Prepare the Flax Egg: In a small bowl, whisk together ground flaxseed and water. Let sit for 5 minutes to thicken.
Cook the Mushroom Mixture: Heat 2 tbsp olive oil in a large pan over medium heat. Add diced onion and minced garlic; sauté for 1 minute. Add mushrooms and cook until softened (about 7–8 minutes). Stir in tomato paste, thyme, cumin seeds, mushroom salt, sumac, smoked paprika, and ground chipotle pepper. Continue cooking until most of the moisture has evaporated. Remove from heat and set aside to cool.
Preheat the Oven: Preheat your oven to 350°F (180°C).
Prepare the Dry Mixture: Add oats and walnuts to a food processor. Pulse until a coarse meal forms (breadcrumb-like texture). Transfer to a large mixing bowl.
Combine the Ingredients: Add the cooled mushroom mixture to the food processor and pulse until combined but not a paste (bits of mushrooms and onions should still be visible). Transfer the mixture to the bowl with oats and walnuts. Add nutritional yeast, flax egg, and a bit of extra dried lemon thyme. Mix well until a wet but rollable consistency forms. If too wet, add more ground oats, 1 tbsp at a time.
Shape the Meatballs: Divide the mixture into 15–20 portions (about walnut size). Roll into balls and place on a baking tray lined with parchment paper.
Bake: Bake the meatballs for 20–22 minutes, rotating the tray halfway through for even cooking.
To serve: Heat marinara sauce in a saucepan over medium heat. Place as many baked meatballs as needed into the sauce and let them simmer gently for 3–5 minutes. Serve over cooked pasta, on toast, or as a topping for your favorite dish. Garnish with vegan parmesan, freshly ground pepper, and fresh herbs of your choice.