Mix the flax and water together thoroughly and set aside. You can also substitute 3 eggs, scrambled together, for this step if you aren't vegan.
Add one cup of the black beans into a large bowl and mash thoroughly.
Add the rest of the ingredients, including the flax mixture (or eggs if using)
Mix ingredients thoroughly, adding salt and pepper to taste.
Form patties with your hands and drop into a preheated non-stick skillet and cook until browned on one side.
Flip and continue cooking. You'll probably cook it a little longer on the second side since the mushrooms will release their water at this stage.
Reduce the heat a smidge and keep cooking till the patties firm back up.
Serve on a soft roll or bun with your favorite condiments