1
In large skillet, heat oil over medium heat until sizzling. Add onions; cook, stirring frequently, 7 to 10 minutes or until onions are softened and golden brown. Stir in vinegar, mustard, salt and pepper. Cook 3 to 4 minutes longer or until glazed.
2
Layer 4 slices toast with lettuce, roast beef and onions. Cover with remaining slices toast.