Preheat oven to 350 and position the oven rack to the center position.

Prep the ingredients by slicing the onions and mushrooms. Cut the peppers in half lengthwise and remove the seeds white membrane.

Turn a burner to medium heat and heat the olive oil. Add the onion, mushrooms, salt and pepper to the pan and cook through, about 8 minutes. Remove the vegetables to the side.

Add more olive oil to the pan, if needed, along with a sprinkle of salt and pepper, and saute the shaved beef for 6 minutes until browned on the outside. It's ok if the inside isn't cooked through since the meat will finish cooking during baking.

Stir in the minced garlic and cook 1 more minute. Add the onion/mushroom mixture back to the pan.

Put the pepper halves in the bottom of a 9x13 baking pan.

Put a slice of provolone cheese in the bottom of each pepper half.

Fill the pepper with the cooked vegetable/meat mixture and sprinkle with a little more salt and pepper.

Place a slice of cheese on the top of each pepper.

Cover the pan with foil, making sure to tent it slightly in the center so the cheese doesn't stick. Be sure the sides of the pan are still sealed.

Bake the philly cheesesteak peppers for 30 minutes and then remove the foil.

Continue baking for 20 more minutes, or until the peppers are cooked through and can be easily pieced with a fork.

Garnish with parsley (optional)

Serve immediately and enjoy the keto philly cheese steak peppers!