Add the beef stew meat and sear on all sides. Lightly season the beef with salt and pepper if desired.
Add onions and saute until lightly browned. Be sure to gently scrape the bottom of the pot to make sure that any browned on bits of meat are not stuck on.
Close the pressure cooking lid and move the valve to the seal position. Pressure cook on high for 18 minutes.
Once the timer is finished, let the pressure naturally release for 10 minutes and then quick release any remaining pressure.
Once the pressure has been released, open the pressure cooking lid.
Stir well. The meat should start to break up into smaller pieces a bit. That's ok. It's actually better for adding to the sandwich.
Turn on saute for 5 minutes, add in mushrooms and green peppers and stir well.
Once vegetables have softened up a bit, add mozzarella cheese and stir well.
Mix softened butter with 1 tablespoon of garlic. Butter the sandwich rolls with garlic butter.
Scoop the beef and vegetable mixture into the sandwich rolls and top with cheese! If desired, you can broil the sandwiches for 5 minutes to fully melt and brown the cheese.