Process the mushrooms, celery, onion, parsley, egg white and soy sauce in a food processor until smooth.
Mix this with the ground chicken and enough bread crumbs to make a soft, but manageable texture.
Divide the mix into 6 and then shape into balls.
On a baking tray lined with paper, press the balls down and place in a preheated oven.
Serve on a muffin half with the lettuce leaf and 2 slices of tomato