Heat oven to 400° F. Cook the couscous according to the package directions. Then stir in 1 tablespoon of the oil and ¼ teaspoon each salt and pepper.
Season the lamb with ¾ teaspoon salt and ¼ teaspoon pepper. Heat 1 tablespoon of the oil in a large skillet over medium-high heat and cook the lamb until browned, about 2 minutes per side.
Transfer the lamb to a baking sheet and roast to desired doneness, about 8 minutes for medium-rare.
Meanwhile, wipe out skillet and heat the remaining oil over medium heat.
Add the onions and cook, stirring occasionally, until soft, about 8 minutes. Stir in the oregano, capers, and ¼ teaspoon pepper.
Divide the lamb and couscous among plates and serve with the onions.