Heat sauté pan over high heat. Add 1 tablespoon butter; melt and add onion slices. Add a pinch of salt and pepper; cook for 3-5 minutes. Remove to plate; reserve. Butter one side of each bread slice. Heat sauté pan over medium heat. Add 4 bread slices, butter-side down. Layer on each slice of bread: 1 slice American cheese, 1 slice Swiss cheese, 2 ounces ham, about 1 tablespoon sautéed onions, 2 slices tomato, and an additional slice of American cheese. Top sandwiches with remaining 4 slices bread, butter-side up. Sauté sandwiches for 3 minutes: turn carefully, and grill until golden brown and the cheeses are melted.