In a heavy skillet, cook onion in butter over medium low heat until onion is tender but not browned. Stir in soup, tomatoes, thyme, and pepper. Add cooked macaroni, ham, and green bell pepper; stir in half of the cheese (1/4 cup). Transfer mixture to a lightly greased 1 1/2 quart casserole. Top with remaining 1/4 cup shredded cheese. Bake at 350° for 30 to 40 minutes, or until heated through.
Macaroni ham casserole serves 4 to 6.