HEAT dressing in large nonstick skillet on medium-high heat. Add onions; cook 2 min. Stir onions and move to one side of skillet.
ADD patties to other side of skillet; cook on medium heat 10 min. or until burgers are done (160ºF), flipping burgers and stirring onions after 5 min.
TOP burgers with VELVEETA; cover. Cook 1 to 1-1/2 min. or until VELVEETA is melted; place on toast slices. Top with onions.