Combine the apple cider vinegar in a cup along with the ground flax seed and a tbsp of water and leave to sit while you prepare the other ingredients.
Put the drained black beans into a bowl and mash with a potato masher to a smooth consistency
Add the finely chopped red onion to the bowl
Blitz the walnuts in a blender until ground and add to the bowl. You can also bash them using a pestle and mortar but its a bit messier with walnuts flying everywhere.
Add the remaining ingredients to the bowl
Scrape the flax and cider vinegar mixture into the bowl
Mix everything in well with a wooden spoon
Put a tbsp of oil into a pan on a high heat
Roll the mixture into balls and place on the pan, then flatten into burger shapes with a spatula
fry in the pan on a medium/high heat for about 5 minutes on each side, until both sides are a little browned
Serve in some wholemeal baps along with your favourite toppings