Sauté onion in butter in a medium saucepan. Remove from heat; stir in tomatoes and salt.
Pour one-third of tomato mixture into a 12- x 8- x 2-inch baking dish. Sprinkle 1/3 cup breadcrumbs over tomato mixture; sprinkle 1 cup Cheddar cheese over breadcrumbs. Repeat layers, ending with tomato mixture topped with 1/3 cup breadcrumbs.
Bake at 350° for 25 minutes. Sprinkle Parmesan cheese over top; bake an additional 10 minutes or until hot and bubbly.