Heat oven to 400° F.
Cook the couscous according to the package directions. Once cooked, stir in 1 tablespoon of the oil, 1/4 teaspoon salt, and 1/4 teaspoon pepper.
Season the lamb with 3/4 teaspoon salt and 1/4 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 to a baking sheet and roast to the desired doneness, about 8 minutes for medium-rare.
Wipe out the 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 1/4 teaspoon pepper.
Divide the lamb and couscous among individual plates and serve with the onions.
Note: Quick-cooking Israeli couscous looks like a grain but is really a pearl-shaped toasted pasta that goes well with roast chicken, fish, and meat. You can usually find it in the rice aisle at the supermarket.