Melt butter in a large skillet. Ad the onions, peppers, and celery, and saute for 3 to 5 minutes. Add tomatoes, sugar, basil,and salt and pepper to taste. Bring to a boil. Mix the carnstarch with vinegar and water, stir into skillet. Pour the mixture into a greased casserole and sprinkle with buttered breadcrumbs. Bake at 350 for 35 minutes.