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