Whisk together all mayo ingredients in a medium bowl. Cover and refrigerate.
Melt 1 tbsp butter in a medium skillet on medium. Add onions and garlic, season with salt and sautee til onions begin to soften. about 3 minutes
Stir in green peppers and mushrooms and sautee til mushrooms brown, another 3-5 minutes.
Turn on your broiler.
Melt 1 tbsp butter in a large frying pan on medium, add beef and toss to coat. Add Montreal steak spice and toss to coat. Continue to cook til meat is no longer red- 3-5 minutes.
Open sausage buns and spread insides evenly with butter. Place under broiler for 30-60 seconds until just beginning to toast. Remove buns from oven.
Place half the beef evenly on each bun, tear the cheese slices in half, place 2 halves on the meat, then cover with vegetables then places 2 more halves on the vegetables.
Return sandwiches to the broiler for 30-45 seconds til cheese is fully melted.
Remove sandwiches from oven, top with mayo and serve.