Preheat oven to 350°. Grease an ovenproof casserole with a lid. In the prepared casserole, combine macaroni, onion, tomatoes, cheese, water, salt and pepper and stir to combine well. Place the lid on the casserole and bake for 1 hour, opening the oven and giving the mixture a stir about halfway through the baking time. Meanwhile, melt butter in a small skillet over medium heat. Add bread crumbs and cook, stirring, for 8 to 10 minutes or until the crumbs are lightly toasted. After the casserole has baked for 1 hour, remove the lid and sprinkle the buttered crumbs over top. Return to the oven, place under the broiler element and broil for 3 to 5 minutes, until browned.