Put portobello mushroom cap in a bowl. Pour balsamic vinegar over the top. Flip it around a few times to coat it. Let it soak for 10 minutes.
Wash and prepare the lettuce, tomato, avocado and onion. Set aside.
Put the chicken patty in the microwave for 90 seconds. Remove and add to burger bun.
Put the mushroom cap on the press for 1-2 minutes, or until lightly browned, Add a piece of cheese and leave it on the press to melt it, about 10-15 seconds. Remove and place on top of chicken patty.