Heat the oven to 400°F. Mix flaxseed and water in a small bowl; let it stand for 5 minutes.
Warm oil in a skillet over medium heat. Cook mushrooms and garlic for 6–8 minutes, until the moisture evaporates.
Pulse walnuts and rice in a food processor until crumbly. Transfer them to a bowl.
Stir in the mushroom mixture, flax mixture, breadcrumbs, seasoning, salt, and pepper.
Shape the mixture into 20 small balls and place them on a lined baking sheet.
Bake for 20–25 minutes, turning once. Warm the marinara and serve it alongside.