Heat 1/4 cup of water in a large frying pan over medium heat. Add the onion and cook until softened, about 5 minutes. Stir in the garlic and then add the beetroot and mushrooms. Sprinkle on the paprika, mustard, cumin, coriander and turmeric. Cook until the vegetables are softened and the liquid is absorbed, about 4 minutes.
In a large bowl, mash the beans well to break them up. Add the cooked grain, flaxseeds and miso. Mash the mixture to combine and then add the oats and walnuts, then the cooked vegetables. Combine until the mixture holds together when pressed between your thumb and forefinger. Divide the mixture into six equal portions and use your hands to shape them into balls. Press the balls into patties and transfer them to a plate. Refrigerate for a minimum of 30 minutes.
Preheat the oven to 375 degrees. Line the baking sheet with a silicone mat or baking parchment and arrange the burgers on it. Bake for 30 minutes, gently flipping the patties about halfway through. Serve hot with or without a bun and your favorite condiments.
Note: Be careful when grating beetroots - their bright red color can stain!