Preheat oven to 400˚F. Heat oil in a large skillet over medium-high heat. Add onion, garlic, ½ tsp. salt and 1/8 tsp. pepper; cook until softened, about 3 minutes.
Add sausage to skillet. Cook until browned, about 4 minutes. Add tomatoes and 1 cup water; cook until sauce is slightly thickened, about 5 minutes. Remove from heat, and stir in couscous.
Fill pepper halves wit couscous mixture. Pour ½ cup water into a large baking dish; arrange stuffed peppers in dish. Cover with foil and bake until peppers are tender, 35-40 minutes. Remove foil and sprinkle with cheese. Return to oven and bake uncovered, 10 minutes.