Create a super sized veggie sandwich by adding sliced tomato and a lettuce leaf or spice it up by spreading chipotle pepper in adobo sauce over the bottom half of the roll.
Use Your Grill
While grilling mushrooms and onions, you can also warm up your bread on the grill. Spread with mayo and top with grilled veggies.
Grill portobello mushrooms and red onions ahead of time; cool and refrigerate. Warm up in microwave when ready to eat.