Top sirloin is the tenderest portion of the sirloin, but it's a large group of muscles that can be broken down further into especially choice value cuts. One of the finest is the petite sirloin, a small, boneless roast cut from the center muscle of the top sirloin. If you cut this roast into steaks -- sold at retail as petite sirloin or "sirloin fillet" steaks -- it's a remarkably fine alternative to costlier cuts from the rib and loin.
The tip sirloin is an extension of the tender loin muscles, bridging the space between the loin's premium cuts and tougher, cheaper cuts from the round. To make a petite sirloin, butchers cut away the cap and "mouse" muscles to leave just the top sirloin center, then cut that into two or three smaller, roughly cylindrical roasts depending on size. Medallions cut from that roast form your petite sirloin or sirloin fillet.
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Tie the steaks with butcher's twine, if you wish, to give them a more regular and compact appearance. Butcher shops often do this before selling the beef because it improves appearance of the roast or steaks and helps them cook more evenly.
Season the steaks as desired. Petite sirloin requires no tenderization and has a rich, beefy flavor, so it doesn't require much help. Simply salting it 45 minutes to 1 hour ahead of time, so the salt can be absorbed into the steak and season it from within, is usually sufficient.
Preheat your grill to 400 degrees Fahrenheit. If you're cooking over charcoal, that usually means a 30-minute wait as your coals burn down and develop a fine coating of ash.
Blot any surface moisture from your steaks with fresh paper towel, then spray or brush them lightly with oil. Petite sirloin can tear badly if it sticks to the grill, and oiling your steaks prevents sticking much more effectively than oiling the grill itself.
Grill your top sirloin for 6 to 10 minutes per side, or until it reaches your desired degree of doneness. If you're more concerned with even cooking than with pretty grill marks, turn the steaks every minute or so for the corresponding 12 to 20 minutes total cooking time.
Test your steaks for doneness by sliding an instant-read thermometer horizontally into the middle. Your thermometer should read 120 to 125 F for a rare steak, 125 to 130 F for a medium-rare steak and 130 to 135 F for a medium steak.
• Steaks of 1 inch or more in thickness benefit from resting for a few minutes under a loose cover of aluminum foil before they're served. Steaks of 3/4 inch or less do not, and can be served straight from the grill.
• Some retailers sell a very different steak, cut from the sirloin tip, under the same "petite sirloin" name. This is a relatively tough cut that benefits from tenderization, and should be handled differently.