HEAT oven to 350°F. 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. Bake, uncovered, 5 min. Let stand 10 min. before serving.