When I worked as restaurant supervisor at the Holiday Inn Sacramento Northeast years ago, this was a favorite among many guests....and one of mine, too!The combination of the balsamic vinegar and mozzarella cheese adds a delightful taste to this vegetarian dish. Even meat-lovers will flip for this meatless burger. Guaranteed to satisfy your next BBQ crowd!
Prepare Zesty mayo Sauce, set aside. Rub Portobello mushroom caps with oil and vinegar,; season each one with a bit of freshly ground pepper, salt and a pinch of fresh chopped parsley. Grill three minutes on one side, then turn and top with cheese slices and onion. Grill until cheese is melted. Put on toasted bun, top with Zesty Mayo Sauce. ENJOY!!!
NOTE: For a little extra zest, sprinkle a bit more Balsamic vinegar on mushroom caps after grilling.