Saute onion in margarine. Add flour and cook 2 minutes. Stir in milk and cook until sauce thickens. Stir in crabmeat, shrimp, parsley, and 1/2 cup cheese. Cook until cheese melts. Pour into buttered casserole dish. Sprinkle with remaining cheese then with bread crumbs. Bake at 400 degrees for 20 minutes or until hot and bubbly through.