In a large nonstick skillet over medium-high heat, melt butter into olive oil. Add mushrooms and onions to the pan; cook until onion is translucent and mushrooms are golden (3-5 min.). Season with salt and pepper. Add beef to pan; heat through (about 5 min.). Spread mustard on cut sides of each roll; top with cheese slices, hot beef and mushroom mixture. Serve immediately.
- Find Kowalski's Roast Beef au Ju in the Meat Department.