Heat oven to 350F. Cook milk, VELVEETA and mustard in large saucepan on low heat until VELVEETA is completely melted and mixture is well blended, stirring occasionally.
Add pasta, bacon, green onions and pepper; mix lightly.
Spoon into greased 2-qt. casserole; cover.
Bake 15 to 20 min. or until heated through; stir. Top with onions.