Sauté onion in shortening in a Dutch oven until tender. Add corn, tomatoes, green pepper, celery, salt, and pepper. Cook over medium heat 10 minutes, stirring frequently to prevent vegetables from sticking.
Soak 2 cups soft breadcrumbs in a small amount of water. Drain well. Add to corn mixture; stir well. Spoon mixture into a well-greased 1 1/2-quart casserole. Sprinkle with remaining 1 cup soft breadcrumbs. Bake at 350° for 20 minutes. Remove from oven, and sprinkle with cheese. Return to oven just until cheese melts.