1. Place mushrooms in a food processor, and pulse until finely ground. Reserve.
2. Heat canola oil in a medium nonstick skillet over medium-high heat. Add sirloin and sausage. Cook until golden brown, breaking up meat with a spoon, about 5 minutes; remove with a slotted spoon and reserve. Add onion to pan and reduce heat to medium-low; cook until soft, about 4 minutes.
3. Preheat broiler.
4. Add mushrooms and red pepper, if desired, to the pan, and mix well. Cook until mushrooms are darker in color, about 5 minutes. Add garlic, and cook 30 seconds until fragrant. Add pasta sauce; stir to blend. Return cooked sirloin mixture to pan; bring sauce to a simmer and cook 2 minutes.
5. Divide mixture among buns and top with mozzarella cheese. Broil in oven or toaster oven until cheese melts. Serve warm.