Step1
Ask the butcher if they will cut the rack into eight equal pieces of rack when choosing the lamb. This will make it easier to marinate the lamb later. If you prefer working with the full rack, you can forego this step. Also ask them to take the membrane off for you.
Step2
Wash the thyme and celery well.
Step3
Mince the thyme finely and set aside the amount you need. Set aside a few sprigs of thyme for garnish.
Step4
Cut the celery leaves and the root bottom off the celery stalks. Slice the celery down the length of the stalk neatly in half, then slice the celery into ½-inch wide pieces. Set aside.
Step5
In a medium saucepan, add the stout and heat over medium-high heat. Bring the stout just to a simmer, then lower the heat to medium-low.
Step6
Add the thyme, celery, peppercorns and onions to the stout. The pearl onions should still have their papery-thin skins on. Cover the saucepan.
Step7
Let the stout simmer for approximately 18 minutes, until the onions are fully cooked and are aromatic.
Step8
Taste the stout mixture and add salt and pepper to your tastes. The stout should taste of onions and thyme.
Step9
Take the saucepan off the heat and let the stout sit until it has cooled.
Step10
Take the lamb ribs and poke the outside of each rib several times with a sharp fork or a paring knife. This allows the flavor of the stout to penetrate the lamb with more ease.
Step11
Place the ribs into a large gallon-sized resealable plastic bag. Pour half of the stout into the bag, ensuring that the ribs are covered fully with the beer. Seal the bag, making sure to press any excess air out of the bag. Set aside the other half of the stout to use for roasting.
Step12
Place the bag of marinating ribs into the refrigerator and marinate the ribs for at least two hours. The longer the ribs marinate, the stronger the flavor will be. Occasionally take the bag out and massage the ribs in the marinade. Replace into the refrigerator.
Step13
Take the bag out of the refrigerator once the lamb has marinated for at least two hours. Discard the remaining marinade in bag.
Step14
Heat the oven to 325 degrees.
Step15
Sit the ribs standing upright in a high-walled roasting pan or casserole dish. Pour the remaining stout over the ribs.
Step16
Place the roasting pan into the oven and cook the ribs for 45 minutes. Check the ribs approximately every 10 to 15 minutes to baste the ribs with stout.
Step17
After 45 minutes of cooking, take the temperature of one of the ribs by sticking your kitchen thermometer into the thickest part of the meat. The temperature should be at least 165 degrees. If it is still not 165 degrees, cook the meat for another 10 minutes.
Step18
Take the roasting pan out of the oven once the meat reaches 165 degrees.
Step19
Serve the lamb ribs with a hearty side, like roasted garlic mashed potatoes.
Step20
Place a few sprigs of thyme on the lamb as a garnish.