Put the beans into a large bowl and roughly mash them with a potato masher, or blend them in a food processor for a few seconds. You want them to keep some texture, so do not over-mash them.
Heat the olive oil (or butter) in a medium frying pan, then sauté the onion for 3 minutes. Add the mushrooms and tamari (or soy sauce) and fry for a few more minutes, until most of the mushroom juice has evaporated.
Stir in the garlic, herbs, spring onion, chilli flakes and mashed beans, then allow the mixture to cook for about 2 minutes, stirring often.
Transfer the mixture to a bowl and leave to cool slightly before mixing in the flour, nutritional yeast flakes (or Marmite) and black pepper to taste.
Divide the mixture into six portions and, using your hands, mould each portion into a patty shape (about 1.5 cm thick and 8 cm wide).
To fry the burgers, heat the 3 tablespoons of vegetable oil in a medium to large frying pan and cook the burgers over a medium heat for about 2 minutes on each side, or until golden brown all over.
If cooking on the barbecue, chill the burgers in the fridge or freeze them for a couple of hours. Then grill on both sides until golden brown and cooked through.