Remove steaks from packaging. Pat dry with paper towels. Sprinkle both sides with the Montreal steak seasoning.
Preheat gas grill (or prepare charcoal fire).
Slice bell pepper into strips, repeat with onion, slicing into strips. Cut mushrooms in half.
Add 1 Tablespoon butter to a cast iron skillet (or large skillet or wok). Place. Place veggies into cold pan. Add a few grinds of pepper and pinch of salt (add more salt if your butter is unsalted)
When grill is very hot, place steaks directly on grill and cover.
Turn on side burner and cook veggies on high, stirring frequently so they do not burn. Turn off heat when they are softened and lightly brown, about 5 minutes. (If you don't have a side burner, delegate this task to someone inside who can cook the veggies, or alternatively use a grill safe pan and cook on grill).
Let steaks cook and don't move them for at least 3 minutes. Check steaks and when they have released (no longer stuck to the grill), and have a nice brown color, flip with tongs. This will probably be 5 minutes, but depends on your grill.
Let steaks cook another 3 minutes and remove for medium rare steaks. If you like it more well done, let cook another few minutes.
Check with a meat thermometer for level of doneness. Add more time to reach your desired temperature.
55 degrees C (130F) - Medium rare
60 degrees C (140F) - Medium
65 degrees C (150F) - Medium well
Remove and place on a clean baking sheet. Let meat rest for 5 minutes covered with foil, or use another baking sheet as a lid.