If you do not have cooked rice, start by cooking according to package directions while you prepare the remaining ingredients.
Preheat your oven to 425 degrees F and line a baking tray with parchment paper.
Add your mushrooms to a food processor and finely chop them, scraping down the sides as needed. If you have a smaller food processor, you may need to do this in batches.
Transfer the mushrooms to a large skillet and dry sauté over medium heat until the moisture and liquid evaporates, approx 6-8 minutes.
Meanwhile, add the onion, garlic, carrot and kale to your food processor and finely chop these as well. Scrape down the sides as needed, and make sure there are no large chunks remaining. Transfer to the skillet with the cooked mushrooms, along with the peas, nutritional yeast and seasoning and sauté another 2-3 minutes, then remove from heat. Add 1.5 cups worth of your cooked rice to the skillet, mix well and taste the mixture and add more salt or other seasoning if desired.
Add the remaining half a cup of your cooked brown rice to a small blender with a tablespoon or two of water, as needed to get it pureed. Once smooth, transfer to the skillet and mix well. Now add the tapioca starch, starting with 5 tablespoons and mix until combined. Form a ball in your hands to test the mixture and see how well it holds. You want the mixture to hold together, but not be overly sticky. If it's very sticky and difficult to form a ball or patty, add another tablespoon or two of the tapioca starch, mix well and test again.
Now divide the mixture into six patties. You'll use a scant 3/4 cup of the mixture per burger. Start by forming a ball in your hands and squeezing gently to ensure there are no gaps. Then flatten in your hands into a patty shape and place down on the baking tray. Do not press the patties into the tray or they may stick while cooking.
Bake for 20 minutes, then remove from the oven and carefully flip each one. Return to the oven for another 10-15 minutes, until the edges are golden brown. Remove from the oven and let them rest at room temperature for 5 minutes before serving.
For added flavour, cook your brown rice in vegetable broth.Make sure your veggies are very finely chopped, but not pureed. Large chunks of veggies will not hold well.When testing the mixture, squeeze some into your hands to see how well it holds together. The pureed rice will make the batter a little sticky, which acts as a binding agent. However, it should not be so sticky that you can't form a ball or patty without the batter completely sticking to your hands. Add more tapioca starch until you reach a slightly sticky consistency. You should need somewhere between five and eight tablespoons. (It will depend on how much water you used to blend your rice.)When forming your patties, do so in your hands and not on the tray. Since there's no oil in this recipe, if you press them into the tray too much, they will stick to the parchment paper. Squeeze the mixture into a ball in your hands, and then flatten into a patty shape and place on the tray gently. If the patties are still sticking to your hands, use slightly damp hands to form them.Leftovers keep refrigerated for 3-4 days and you may reheat in the oven or in a skillet on stove top.To freeze, let the patties cool and then place in a freezable container with your patties layered between sheets of parchment paper to prevent sticking.