Preheat oven to 400°F/200°C. Lina a baking sheet with parchment paper and spray a large skillet with vegetable cooking spray.
Place potato in the microwave and cook for 2 minutes. Use oven mitts to remove from microwave and allow to cool about 5 minutes. Once cooled, chop into large cubes.
Place oats and walnuts in a food processor and pulse until a coarse flour is formed. Add potato, chickpeas, onions, garlic powder, poultry seasoning, nutritional yeast, and 1 tablespoon ground flaxseed. Pulse 3 seconds, then use a spatula to push ingredients back down. Pulse again for 3 seconds.
Form mixture into 6 patties and place in a prepared skillet over medium heat. Cook 5 minutes on one side. Flip, then sprinkle salt and pepper on uncooked sides and cook for another 5 minutes until patties have firmed up slightly and are brown on both sides. Transfer from skillet to a large plate and set aside to cool for about 5 minutes.
In a small bowl stir together bread crumbs and vegan Parmesan. Pour melted butter, milk, and the remaining 1 tablespoon flaxseed into a separate small bowl. Dip each patty in butter mixture, then dip in breadcrumbs and place on a prepared baking sheet. Spray patties lightly with vegetable cooking spray.
Bake 25 minutes until bread crumbs are golden brown. Remove from oven.
Spoon equal amounts of marinara sauce over each patty. Top with equal amounts of mozzarella shreds. Return pan to oven and bake another 10 more minutes until cheese is melted.
Let cool for 10 minutes, then transfer patties to a large lidded container and refrigerate for up to 10 days or freeze for up to 2 months. To serve, cook in the microwave in 30-second intervals until heated through.