Roast beef is a fine meal, but it takes time. Doing one on the weekend and using the uneaten portion for a couple of easy weekday lunches or weeknight meals is an obvious way to get the best value for the time and money you've put into that roast. You can opt to reheat the roast beef in gravy, if you like, or you can do it without gravy if that's a better option for the meal you have in mind. You'll just need to choose a method that won't dry the meat.
Lack of Gravy
Reheating roast beef in a pot of simmering gravy is a simple and effective way to warm the meat to serving temperature without drying it out. It's not always going to be an option, though, for many reasons. The most obvious is that you just plain might not have any gravy left on hand. That can certainly happen, especially if you've got a real gravy lover at the table or if you've already used what was left to reheat other leftovers. You might also just want to use a different sauce or make a dish that wouldn't work well with gravy.
What Kind of Roast Beef?
The kind of roast you're reheating also has a bearing on your choice of a warming method. If you start with a slice of delicately pink prime rib, then, ideally, you'd like to finish with it looking much the same. If you're working with a piece of pot roast that's already falling apart, maintaining its degree of doneness won't be your issue. Instead, you'll be concerned more about the fork-tender meat becoming dry or chewy.
Then there's the question of what form your leftover beef takes. If you've left the remainder of your roast as a single solid piece, it will take much longer to reheat, but there's less risk of it drying out. If you're working with slices, they'll reheat more quickly, but you'll have to be careful about their getting overheated and dry. It's also harder to maintain a given degree of doneness with slices, especially if they're thin. When you choose your heating method, keep those considerations in mind.
Reheating a Roast in Your Oven
Reheating beef in the oven can be the simplest option if you have a large piece of roast to reheat ‒ a pound or more in size. The best method is to place a wire rack on a baking sheet and then place the roast on the rack, so air can circulate all around it. Warm the roast uncovered at 250 degrees Fahrenheit until it returns to about 120F when tested with an instant-read thermometer, which will take an hour or more, depending on the size and shape of the roast. Finish the roast by patting it dry and searing the already browned sides — but not the cut ends — for a minute or two until they're browned.
Other Options for a Whole Roast
If you'd like to keep your oven free for other uses or if you just don't want to warm up your kitchen that much, you can reheat a whole roast almost as well using a couple of other methods. If you have a countertop roaster oven, you can use it exactly the same way: Place a rack in the bottom and reheat your roast beef; then sear it to finish. If you have a large countertop toaster oven or rotisserie, one of those will also work. You'll just have to keep a close eye on the roast and turn it as needed in a toaster oven, because the heating elements are closer to the meat and might burn it.
Warming Sliced Beef in a Bag
The best way to heat up sliced roast beef may be inside a sealed bag. Vacuum-sealed bags are ideal if you have a sealer; those bags are airtight and won't leak. If you don't have a vacuum sealer, put your portions of roast beef into a good-quality zipper-seal bag and squeeze out the excess air as you seal it. To be extra careful, double-bag the beef to protect further against leaks.
Place the sealed bag or bags in a pot filled with water at a low simmer or in a large bowl with freshly boiled water. If you opt for the bowl, top it up periodically with hot water to maintain a relatively even temperature of 180F or so. If you're heating just a portion or two and it's sliced thinly, it should only take a few minutes for the beef to reach serving temperature. Lift the bag from the water and squeeze it with your fingertips to judge the heat. If it feels hot enough to eat, it probably is. Thicker slabs will take longer, and they should be tested with a thermometer.
Warming in a bag is gentle enough to keep your medium-rare slab of prime rib medium-rare, and — because the package is sealed — your loss of moisture should be pretty close to zero. That makes it a nearly ideal technique to reheat roast beef.
Rewarm in a Steamer
A second option that gives excellent results and won't dry out your meat is reheating in a steamer. Set up your steamer as usual, whether it's a pot on the stovetop or a standalone countertop unit. Wrap your sliced beef or your slab of prime rib tightly in foil. If you don't want the meat coming into contact with the aluminum foil, you can line it with parchment first.
Heat the beef for 3 to 4 minutes if the slices are thin or up to 6 or 7 minutes for a thick piece of prime rib. Take the pouch from the steamer and test the temperature with your fingers or an instant-read thermometer. If it's not quite ready, put it back for another minute or two. This technique also maintains your degree of doneness pretty well, especially for thick slices, and it helps keep your beef from drying out.
Warming Slices in the Oven
If you just want to pop your beef in the oven and forget about it for a while or if you're reheating a large quantity of roast beef, you may need to use your oven. The oven can dry out meat, so you'll have to take steps to counter that possibility. It's also harder to keep a given degree of doneness this way, so it's best to use this method only when you're reheating pot roast or a roast that's already medium to well-done.
Arrange your sliced beef in the bottom of a baking dish, ideally in a single layer. Drizzle it with a few tablespoons of water or broth to moisten it and create steam; then cover the dish with its lid or a tightly wrapped sheet of foil. Heat gently at 250F until the slices are heated through, for 8 to 15 minutes, depending on how much beef you're warming and how thickly it's sliced. The same basic technique works equally well in a toaster oven or a countertop roaster as long as you've got a baking dish that fits.
Warming Slices in the Microwave
A microwave oven is a chancy way to reheat roast beef, because it cooks unevenly and can dry out your slices in a hurry, but it's a useful option for reheating small quantities. Again, arrange your slices in a single layer and add a splash of water or broth to provide steam, which helps the beef heat more evenly. Cover the dish with its lid or with plastic wrap, and microwave it for a minute and then rest it for 2 minutes. Uncover and check the meat's temperature, and repeat if necessary.
Leftovers, Food Safety and Quality
No discussion of leftovers is complete without a few words about food safety. Any time you plan to cook for more than one meal, whisking what's left on the table or stovetop into your fridge as soon as possible is a crucial step in the process. Your roast beef should never be at room temperature for longer than two hours before it gets to the fridge, but sooner is better. You don't want to give bacteria any more time to grow than you absolutely have to. Packing in airtight containers or vacuum bags as soon as it's cool can also help maintain the best flavor and quality.
The USDA advises heating all leftovers to 165F for maximum food safety, which is especially prudent if you're elderly or pregnant or your immune system is compromised. It's not necessarily best for eating purposes, though, especially if you're picky about having your beef at no more than medium-rare. Heating your beef back to its original temperature isn't especially risky, if you've taken care to follow safe food handling practices from start to finish. Like rare steaks or soft-cooked eggs, which the USDA also discourages, it's a judgment call for the individual cook or diner.