In a large skillet, cook onion in 1 tablespoon butter over medium heat for 8-10 minutes or until golden brown, stirring frequently.
Spread remaining butter over one side of each slice of Texas toast. Place slices, buttered side down, on a griddle. Spread with mustard; layer with ham, apple, onion and cheese. Top with another slice of toast, buttered side up. Cook over medium heat for 3-4 minutes or until golden brown. Carefully turn; cook 2 minutes longer or until cheese is melted. Yield: 4 servings.