Preheat broiler. In a 6-qt. stockpot, heat butter over medium heat. Add mushrooms, onion and peppers; cook and stir 5-7 minutes or until tender. Add garlic; cook 1 minute longer. Drain vegetable mixture; stir in salt and pepper.
Arrange buns on a baking sheet. Layer bun bottoms with pastrami and vegetable mixture; top with cheese. Broil 4-5 in. from heat 1-2 minutes or until cheese is melted and bun tops are toasted.
Every editorial product is independently selected, though we may be compensated or receive an affiliate commission if you buy something through our links. Ratings and prices are accurate and items are in stock as of time of publication.