Heat grill to medium heat.
Grill franks 7 to 9 min. or until heated through, turning occasionally.
Meanwhile, cut VELVEETA into 1/2-inch cubes; place in microwaveable bowl. Microwave on HIGH 1 min. or until completely melted, stirring after 30 sec.
Fill buns with franks; top with chili, VELVEETA and onions.