ADD garlic, mushrooms, and walnuts to a food processor and pulse a few times until finely chopped. Be careful not to chop the veggies too finely so that they start to get mushy – you want small pieces so they add texture. Transfer to a bowl.
ADD onions to a food processor and pulse a few times to finely chop.
HEAT olive oil in a skillet over medium heat. Add onions and a pinch of salt and cook, stirring frequently, until lightly browned, 3-5 minutes.
ADD pulsed mushrooms, walnuts, garlic, and a pinch of salt. Cook, stirring frequently, until mushrooms have softened, about 3 minutes. Remove from heat and transfer the veggies to a large mixing bowl. Let the veggies cool while you prepare the rest of the filling.
SQUEEZE excess water out of thawed tofu. Tear tofu into chunks and add to a food processor. Process until tofu is broken up into small crumbles.
ADD remaining burger steak ingredients to a food processor and pulse a few times to combine - again leaving some bigger pieces for texture.
TRANSFER the mixture in the food processor to the bowl with the veggies. Mix until well combined. If the mixture is still quite wet and not holding together, add another tablespoon of potato starch.
COVER the bowl and rest in the fridge for 1 hour.
PREHEAT oven to 400°F while you shape the burgers.
DIVIDE burger mixture into 9 pieces. Shape each piece into a burger.
PLACE burgers on a parchment-lined baking sheet and lightly spray or brush the burgers with some olive oil.
TOSS potatoes, zucchini, and broccoli with olive oil, garlic powder, salt, and pepper. Spread out on a separate baking sheet.
PLACE both baking sheets into the oven and bake for 25 minutes, until burgers are browned and veggies are tender.
MAKE gravy while the burgers cook. Heat olive oil in a deep pan over medium heat. Add onions and a pinch of salt and cook until they start to brown.
ADD mushrooms and cook until softened.
STIR in nutritional yeast and thyme and cook for 1 minute.
MIX together vegetable broth, tapioca flour, and tahini. Pour into pan and simmer until thickened, stirring frequently. Turn off heat.
WHISK together miso and water. Stir into gravy.