Slice hoagie rolls in half horizontally, but don't cut all the way through the roll.
Spread butter onto the cut sides of each roll. Set aside.
Slice onion. Set aside.
Seed and slice green bell pepper. Set aside.
Slice sirloin steak into ΒΌ inch thick strips. Set aside.
Preheat grill over medium heat.
Place hoagie rolls onto the grill, with the cut/buttered sides down. Cook until toasted and lightly browned. Remove toasted rolls from the grill and set aside.
Add a couple tablespoons butter to the hot grill.
Add sliced onion and cook until soft, translucent and beginning to brown.
Remove onion from the grill, set aside.
Add another tablespoon of butter to the grill.
Add the green bell pepper slices and cook for 1-2 minutes until crisp tender. Remove from the grill, set aside.
Add another tablespoon butter to the grill.
Add the sliced beef strips and season generously with salt, pepper and onion powder as they cook.
Cook beef to medium rare doneness tossing the strips with a spatula as they cook.
Return sliced onions and peppers to the grill and toss with the beef strips.
Use large spatula too create three long even piles of the meat/onion/pepper mixture.
Top each pile with slices of cheese and cook until cheese melts.
Top each pile with a hoagie roll, toasted sides down.
Use spatula to scoop up the filling and transfer the sandwiches onto a plate flipping them over so the filling is on top.
Serve sandwiches while they are nice and hot and cheese is melty.