In a shallow dish lay your mushrooms and pour in soy sauce and pineapple juice. Cover and allow to marinade for at least 2 hours. Turn often to ensure that all side are evenly coated.
In a large cast iron, add a splash of olive oil and your thinly sliced onion, allow to soften on one side before flipping over.
Add the mushrooms and the pineapple rings and ¼ of the marinade. Flip once pineapple has begun caramelizing and the mushroom has softened. Add more liquid as needed to prevent sticking.
While the mushrooms are cooking prepare your toppings. Toast your buns and in a bowl combine the Vegenaise and finely chopped garlic. Slather on one side of the toasted buns.
Add the mushroom, onions, pineapple, lettuce and avocado and sink in.
