Heat oven to 400 degrees F. Bring 3 cups of water to a boil. Add the couscous and cook until tender, 8 to 10 minutes. Drain and run under cold water to cool. Meanwhile, heat 1 tablespoon oil in a large oven-safe skillet over medium-high heat. Season the lamb with the cumin and 1/2 teaspoon each salt and pepper; cook until browned, 2 to 3 minutes per side. Transfer the skillet to the oven; roast to desired doneness, 4 to 6 minutes for medium-rare. Using a vegetable peeler, remove 2 strips of lemon zest. Thinly slice the zest. Squeeze 2 tablespoons lemon juice into a large bowl. Stir in the zest, remaining 2 tablespoons oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Add the cooled couscous and toss to coat. Fold in the arugula and serve with the lamb.