In a small bowl, whisk water and ground flaxseeds together. Set aside.

Combine onion and toasted walnuts in a food processer and pulse until finely chopped.

Add oats, seasonings and flaxseed mixture, and pulse until smooth. Stir in mushrooms.

Using 1/4 cup of batter, shape into patties and fry in a lightly oiled skillet until browned.