01 - Preheat oven to 400 degrees Fahrenheit. Line a baking sheet with parchment paper.
02 - Heat 1 tablespoon olive oil in a large skillet over medium heat. Sauté diced onion for 3 to 4 minutes until translucent. Stir in minced garlic and cook for 1 minute longer.
03 - In a food processor, add cooked lentils, sautéed onion and garlic, chopped walnuts, breadcrumbs, flaxseed mixture, soy sauce, thyme, allspice, nutmeg, black pepper, and salt. Pulse until mixture is cohesive and slightly textured.
04 - Using your hands, form mixture into 20 to 24 small balls and arrange evenly on the prepared baking sheet.
05 - Bake meatballs for 18 to 20 minutes, turning halfway through, until firm and lightly golden.
06 - Melt vegan butter or heat olive oil in a saucepan over medium heat. Whisk in flour and cook for 1 to 2 minutes until pale golden. Gradually whisk in oat milk or soy milk, followed by soy sauce, Dijon mustard, white pepper, salt, and onion powder. Simmer and whisk frequently until sauce is thickened, approximately 5 to 7 minutes.
07 - If desired, heat a small amount of olive oil in a skillet and pan-fry baked meatballs briefly until evenly browned.
08 - Transfer meatballs to gravy and gently toss until fully coated. Serve immediately alongside mashed potatoes, lingonberry jam, or steamed greens.