Gather your ingredients and Pre heat the oven to 350 degrees.
Wash & core out the seeds of the peppers.
In large non stick pan, spray with some olive oil and sauté the chopped onion and sliced mushrooms. Season is Sea Salt & Pepper to taste.
Place you peppers into a baking dish. Fill each pepper with 1 slice of provolone cheese.
Slice your Roast Beef into strips.
Set aside the mushrooms & onions and in the same pan, heat the beef just for a quick moment to warm.
Toss together the warmed beef with mushrooms & onions.
Fill each pepper with the beef mixture until all used and evenly distributed.
The peppers are Filled to the brim and ready for the topping.
Top each pepper with 1 slice of provolone cheese. Put into a Pre heated oven and cook for 35 minutes.
Done, peppers are tender & cheese crispy! Ready to serve warm... Enjoy!