Place bacon in a large skillet; cook and stir over medium-high heat until evenly browned, about 10 minutes. Add mushrooms, green bell pepper, onion, and garlic; cook and stir until mushrooms are softened, about 10 minutes. Remove skillet from heat.
Preheat an outdoor grill for medium-high heat and lightly oil the grate.
Place 1 round steak between 2 pieces of plastic wrap on a flat work surface. Pound steak using a meat tenderizer until flattened. Repeat with remaining steaks.
Pour Montreal steak seasoning into a shallow bowl. Press 1 side of each steak into the seasoning and lay seasoning-side down on a cutting board. Spoon some of the mushroom mixture down the center of each steak. Roll up steaks and secure with toothpicks.
Cook on the preheated grill, turning occasionally, until cooked to desired doneness, about 35 minutes. Remove toothpicks before serving.