Preheat your oven to 200 C.
Roast beef, this is supposed to be deli sliced roast beef, now unless you have you have a big budget, there is no need to go Villa Market or Rimping in Chiang Mai, make a nice roast beef dinner the day before, use some beef the next day for this recipe. I will not figure roast beef into this recipe.
In a non stick pan heat some olive oil and saute the onions, garlic, and mushrooms (if using) and the diced the tops of the peppers and cook for 5 to 10 minutes on medium heat or until lightly browned & tender, then add the sliced roast beef strips & steak seasoning and stir and cook until heated through.
Place the bell peppers in a baking dish, add a slice of Provolone cheese to the bottom of each pepper. Then add the beef mix to about halfway up each pepper, add another slice of cheese, then more beef mixture, then top with another slice of cheese, tucking the edges of the cheese into the pepper.
If you have more beef filling than peppers, you can do one of two things, cut the top off another pepper, or two, and fill those, or just pack the meat around the peppers in the baking dish.
Bake for 15 to 30 minutes or until the peppers are cooked through. Serve with some pasta or boiled potatoes on the side.