01 - Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.
02 - Heat 1 tablespoon olive oil in a large skillet over medium heat. Sauté onion for 3–4 minutes until translucent, then stir in garlic and cook for 1 minute longer.
03 - Combine cooked lentils, sautéed onions and garlic, walnuts, breadcrumbs, flaxseed mixture, soy sauce, thyme, allspice, nutmeg, black pepper, and salt in a food processor. Pulse until combined and mixture holds together but retains some texture.
04 - Shape the mixture into 20–24 small balls and arrange them on the prepared baking sheet.
05 - Bake for 18–20 minutes, turning halfway through, until the meatballs are firm and lightly browned.
06 - Melt vegan butter or heat olive oil in a saucepan over medium heat. Whisk in flour and cook for 1–2 minutes until lightly golden.
07 - Gradually whisk in oat or soy milk, then add soy sauce, Dijon mustard, white pepper, salt, and onion powder. Simmer, whisking frequently, until thickened, about 5–7 minutes.
08 - If desired, pan-fry the baked meatballs in a little olive oil for enhanced color.
09 - Add meatballs to the gravy and gently toss to coat. Serve hot, ideally with mashed potatoes or lingonberry jam.