Cook onion and peppers in a Multipurpose Steamer for 2–3 minutes or until soft.
Sprinkle both sides with Montreal Steak Rub. Grind Herbed Garlic Sea Salt Blend, if desired.
Grill steak over medium-high heat. For medium-rare, cook about 3–4 minutes per side. Remove from grill, let stand 5 minutes, slice thinly and sprinkle with cheese.
Slice pretzel buns, and spread Chipotle Aioli on bottom of bun. Top with sliced steak and cheese, BBQ sauce, peppers, lettuce and onions. Serve immediately.