Heat grill to medium heat.
Mix first 4 ingredients just until blended.
Shape into 8 (1/2-inch-thick) patties.
Grill 5 to 6 min. on each side or until done (160ºF). Top with Singles; grill 1 min. or until melted.
Place lettuce, tomatoes and onions on bottom halves of buns; cover with burgers, ketchup and tops of buns.