1Place 1/4 cup cheese on each of 4 bread slices. Divide onion, bacon, tomato and avocado evenly onto bread slices. Top with remaining cheese and bread slices. Spread 2 teaspoons butter over each top slice of bread.
2Place sandwiches, buttered side down, in 12-inch skillet over medium heat. Spread remaining butter over top slices of bread. Cook uncovered about 5 minutes or until bottoms are golden brown. Turn; cook 2 to 3 minutes longer or until bottoms are golden brown and cheese is melted.