Place pepper halves, cut sides up, in 13x9-inch baking dish. Bake 10 min.
Meanwhile, cook onions in dressing in large nonstick skillet on medium heat 5 min. or until onions are crisp-tender, stirring frequently. Spoon onions into bowl. Brown meat in same skillet; drain.
Prepare stuffing as directed on package. Add to meat along with the onions; mix lightly. Spoon into pepper halves; top with VELVEETA. Cover.
Bake 15 min. or until VELVEETA is melted and filling is heated through.