1/2 lb. Velveeta, cubed
1/2 c. Kraft Real Mayo
1/4 c. milk
1 tsp. dry mustard
1 med. onion, sliced
2 tbsp. butter
1 lb. thin roast beef slices
6"-8" French bread rolls, split

Combine process cheese spread, mayo, milk and mustard. Stir over low heat until smooth. Saute onions in butter. Add meat; mix lightly until thoroughly heated. Fill rolls with meat mixture. Top with sauce.