Heat grill to medium heat.
Mix first 3 ingredients just until blended; shape into 4 (1/2-inch-thick) patties.
Grill 4 to 5 min. on each side or until done (160ºF). Top with Singles; grill 1 min. or until melted.
Place lettuce on bottom halves of buns; top with cheeseburgers, onions and ketchup. Cover with tops of buns.