- Heat grill to medium heat.
- Mix meat and 2 Tbsp. steak sauce; shape into 4 (1/2-inch-thick) patties. Spray both sides of onion slices with cooking spray.
- Grill patties 5 to 7 min. on each side or until done (160ºF), adding onions to grill after 2 min. and cooking until crisp-tender. Top burgers with 2% Milk Singles; grill 1 min. or until melted.
- Fill buns with burgers, tomatoes, onions and remaining steak sauce.