Preheat your oven to 350F and line a small baking sheet with parchment paper or a nonstick baking mat.
Halve your eggplant lengthwise and coat lightly with netural oil. Season with 1/2 tsp salt and place cut side down on a sheet pan. Roast for 25-30 minutes, until flesh is tender and soft. Scoop out the flesh and discard the skin. Add the flesh to a food processor and process until smooth. Set aside in a large mixing bowl.
Pulse chickpeas in a food processor until roughly chopped, then add to the same mixing bowl. Add the cashews and the cooked buckwheat.
In a skillet or heavy bottomed pot or pan over medium high heat, add a generous amount of oil to cover the bottom. Once sizzling, add leeks, celery and carrot and sauté until soft (roughly 5-8 mintutes) then add onion and cook for an additional 1-2 minutes. Add tomato paste and cook down, stirring often, until the tomato paste turns a deeper red colour. Remove the veggies and set aside in another bowl.
In the same pan, add enough oil to cover the bottom and over medium high heat, cook the mushrooms. Let sit without stirring for about 5 minutes, until edges are golden brown. Stir to cook and transfer the mushrooms to the food processor. Add the cooked vegetable mixture and food process until broken down but not entirely smooth.
Transfer the vegetable mushroom mixture to the large mixing bowl and add potato starch, nutritional yeast, miso paste, tamari, cumin, garlic powder, paprika, liquid smoke, 2 tsp salt and pepper. Use a spatula to mix well and let the mixture chill in the fridge for at least 30 minutes or until chilled and slightly more firm.
To form the veggie patties, scoop about 1/2 cup of the mixture and roll it into a ball. Coat the ball with the breadcrumb mixture and press into a patty shape. Gently lay onto a lined baking sheet and repeat to form the remaining patties. If the mixture feels wet, feel free to add additional potato starch or breadcrumb mixture and stir it in. I like to store prepared patties in the freezer.
To cook a patty, heat 2-3 tbsp neutral oil in a large skillet over medium heat. Add the patty and cook for 3-4 minutes on each side, turning gently with a spatula, until golden brown.