Pulse the oats and walnuts in a food processor until they are finely ground then add the onion, mushrooms, beans, tahini/almond butter and flax. Pulse to mix well. Add the remaining ingredients and mix until well combined.
Test if the mixture is holding together well by pinching it. If it is too wet, add more oats, if it is too dry, add a little water a tablespoon at a time. Transfer the mixture to work surface, divide into 4 equal portions and shape each portion into a burger patty about 1cm thick. Place on a plate and refrigerate for about 30min.
Preheat the oven to 180C / 350 F.
Line a baking sheet with baking parchment and bake the burgers until hot and lightly browned, turning once – about 30 – 40min. Serve in a wholemeal or GF bun with salad and a side of sweet potato wedges.