Preheat the oven to 400°.
Slice the peppers lengthwise and remove any seeds and ribs.
Spray a 9x13 casserole dish lightly with your choice of non-stick cooking spray. Place the peppers in the dish.
Bake the peppers in the dish, uncovered, cut side up, for 15 minutes.
While the peppers are in the oven, prepare the filling. Place a large skillet on the stove, over medium-high heat. Allow the pan to warm up for a few minutes.
Add the butter, and once melted, add the onions and mushrooms. Saute for about 5 minutes, occasionally stirring, just until the juices from the mushrooms release.
Add the garlic and stir until fragrant, about 30 seconds.
Remove the onion mixture from the pan and set aside.
Season the steak with salt. Add it to the hot pan and sprinkle the steak seasoning over it.
Stirring occasionally, cook the meat for about 3 minutes.
Add the onion mixture back into the pan and combine.
Once the peppers are finished cooking, and your filling is ready, place 1 piece of provolone cheese over each pepper, pushing it down so it forms to the pepper.
Add filling, fill generously.
Top each pepper with another slice of cheese. Place under the broiler for about 3 minutes, until the cheese is melted and bubbly. (watch closely).
Garnish with parsley and serve.