Step 1
Stir beef Worcestershire sauce and Montreal steak seasoning together in a bowl. Marinate beef while preparing other items.
Step 2
Heat olive oil in skillet over high heat. Cook and stir onions in hot oil until golden brown about 15 minutes; add balsamic vinegar salt and black pepper. Reduce heat to low and cook the onion mixture at a simmer until there is little liquid remaining and the mixture is syrup-like in texture about 10 minutes.
Step 3
Preheat an outdoor grill to 400 degrees F (200 degrees C) and lightly oil the grate.
Step 4
Remove steak pieces from the marinade and shake off excess. Discard the remaining marinade. Combine beef and canola oil in a bowl and toss to coat.
Step 5
Preheat oven to 200 degrees F (95 degrees C).
Step 6
Cook the beef slices on the hot grill turning frequently until they start to firm and are reddish-pink and juicy in the center 4 to 6 minutes per side. An instant-read thermometer inserted into the center should read 130 degrees F (54 degrees C).
Step 7
Arrange rolls onto a baking sheet. Spoon onions into the bottom of the sandwich rolls; add beef. Top beef with Brie cheese.
Step 8
Heat in preheated oven until the cheese melts about 5 minutes.