Preheat oven to 250 degrees F.
Heat a non-stick skillet with 1 tablespoon butter on medium high heat.
Add onions and cook for 5 to 8 minutes or until onions are softened and translucent.
Season the steak pieces with salt & pepper.
Add to the skillet and cook for 2 to 4 minutes or until just a little bit of pink remains.
Divide the mixture in half, pushing each half to one side of the skillet.
Place 2 slices of cheese on top of each half; cover and let melt for 1 to 2 minutes.
Carefully transfer each half of the meat mixture to the inside of the split hoagie rolls.
Wrap tightly with aluminum foil; place on a baking sheet and bake for 10 to 15 minutes or until warmed through.
Serve immediately.
