Pre-soak red (kidney) beans for 4-5 hours or overnight. You can also use canned beans to simplify. Wash them and cook until soft but not mushy.
Cook peeled potatoes. Use 1 large potato (or 2 small ones) for the burger and keep the rest for the patties.
Separate mushroom stems from caps. Fry the stems with onions for 3-5 minutes. We will use these for ground-beans-"meat". Slice the caps and fry them with onions for 4-5 minutes. These will be our burger "decoration" or a layer.
Add beans, fried mushrooms (the stems), a few cooked potatoes, walnuts, spices, and homemade crouton crumbs (optional) to the food processor and mix.
Form the burgers and put them into fridge for 20-30 minutes.Grill for 1-2 minutes on each side.