Heat oil in a large skillet over medium heat; add onions, peppers and salt.
Cook vegetables over medium-high heat until tender, about 10 minutes.
Add beef to skillet.
Saute for 2 minutes for medium-rare.
Remove beef and vegetables from skillet with a slotted spoon; drain.
Divide beef and vegetables evenly among sandwich rolls.
Top with cheese.
Loosely wrap sandwiches in foil.
Bake until cheese melts, about 10 minutes.
PS:Philly Cheese Steak Sandwich tip: Reduce your cleanup time by microwaving the onions and bell peppers. Place in a resealable plastic bag with 1 tablespoon oil and microwave on HIGH for about 4 minutes.