In large skillet heat olive oil over medium-high heat. Add onion slices and cook 8 to 10 minutes or until very tender, stirring frequently.
Meanwhile, prepare burgers according to package microwave directions.
In small bowl combine mayonnaise and mustard; spread mixture on one side of each bread slice. Top bread with spinach and burger. Stir steak sauce into cooked onions. Spoon onion mixture on burgers. Makes 4 servings.