Rinse under cold water and pat dry with paper towels. Let sit for 45 minutes to 2 hours at room temperature.
To make the marinade; combine all the ingredients in a bowl; whisk well and set aside.
Preheat oven to 325ºF. Cover the bottom of a roasting pan with chopped onions. Place lamb on top and pour the marinade over applying evenly. Pour chicken broth around the meat to keep the meat moist during cooking. Insert a probe into the thickest part of the lamb without touching the bone. Cover tightly with foil and transfer to the preheated oven. Roast lamb for 2 to 2 ½ hours depending on how you like it cooked.
If medium is your preference, remove foil when the internal temperature reaches 145ºF and cook until the internal temperature reaches 160ºF. Remove from the heat and transfer leg of lamb to a serving dish.
Pour the pan juice into a saucepan and bring to a boil. Add cornstarch mixture, stir and bring it back to a boil until the sauce thickens. Serve along with lamb and couscous. Makes 8 servings.
TEMPERATURE CHART (internal temperature)+.
Medium-Rare = between 140ºF to 150ºF which is about 20 minutes per pound.
Medium = between 150ºF to 160ºF which is about 25 minutes per pound.
Medium Well = between 160ºF to 170ºF which is about 30 minutes per pound.
Well-done = 170ºF + which is about 35 minutes per pound.