Philly-style cheese steak sandwich

1. : thinly slice the roast been and heat until warm.
2. Sautei the onion until limp in a skillet coated with no-stick cooking spray.
3. Meanwhile, tear the cheese slices into small bits and place them in a measuring cup with the milk.
4. Melt on a stove in a saucepan, stirring twice.
5. Layer the roast beef on the roll, followed by the onion and the cheese sauce.
6. Wrap in foil and warm in a 350°F oven for several minutes.