Add walnuts to the food processor and grind until nuts resemble a fine crumb.
Add mushrooms and continue to pulse until finely chopped.
Add remaining ingredients, except butter, and blend until well combined.
Add butter and blend until mixture is completely smooth.
Shape mix into 8 burger shaped round patties using clean hands and place on a lined baking tray.
Place the tray in the fridge for 1 hour, or in the freezer for 25 minutes. These patties can be made ahead of time up until this point and kept covered in the fridge until ready to cook. You can also freeze any extra uncooked burger patties to use next time.
Heat 1 tablespoon of olive or coconut oil in a frying pan. Carefully place the patties on the pan, cooking in batches so as to avoid overcrowding the pan.
Fry the patties for 2-3 minutes on each side, until brown and crispy.
Once both sides of the patties have been cooked, place one slice of vegan cheese on top of each pattie, while they are still in the pan. Cover the pan with a lid or baking tray and transfer the entire covered pan into a preheated oven for 5 minutes, or until the cheese has melted.
Cut the burger buns in half. Spread each half with 1 tsp cheeseburger sauce.
Top with diced onions, burger patties, tomato, pickles and lettuce. Serve immediately with fries.