Add the cashews to a food processor and pulse to form a crumb consistency.
Add the cooked potato, parsley, onion, garlic, black beans, cumin and flax to the food processor. Season to taste and pulse to combine to a thick, slightly chunky paste.
Transfer the burger mixture to a bowl, cover and refrigerate for 30 minutes to firm up.
Preheat the oven to 200 degrees conventional or 180 degrees for fan assisted and line a shallow oven tray with baking paper.
While the mixture is firming, whisk together the tahini, olive oil and salt and reserve to one side.
Remove the burger mixture from the fridge and divide into two equal portions. Using your hands, form into 2 even patties and place on to the lined oven tray.
Place in the oven to bake for 18 minutes turning halfway through, until piping hot and golden brown.
While the burgers are cooking, preheat a grill to high and brush the mushrooms with a little olive oil.
Add the mushrooms to a grill tray and grill for 5-7 minutes until cooked and tender.
To assemble the burger, lay one mushroom cap topside down and layer with half the lettuce. Top with one burger patty, followed by the sliced tomato. Top with the second burger patty and the sliced onion. Drizzle over the tahini sauce and top with the remaining lettuce and 2nd mushroom cap.