Saute or caramelize the onions, cooking over medium heat with a small amount of oil
Cut each pepper in half lengthwise, remove the pulp and seeds
Line each pepper half with a slice of provolone cheese
When onions are about done, toss in the mushrooms with some pepper, cooking for 1-2 more minutes
Add the roast beef with a pinch of salt, stir, and cook for another 1-2 minutes
Stuff each pepper half with the beef mixture
Place a slice of provolone cheese on top of each pepper half
Bake at 375 for about 20 minutes