Sauté the Vegetables. In a skillet, heat a little olive oil. Add chopped mushrooms, onions, and garlic. Cook until softened and most of the moisture has evaporated, about 6–8 minutes.
Make the Flax Egg. In a small bowl, combine ground flaxseed with water. Let sit for 5–10 minutes to gel.
Mix the Meatballs. In a food processor, combine the sautéed vegetables, cooked lentils, oats, breadcrumbs, flax egg, soy sauce, and seasonings. Pulse until well mixed but not completely smooth. You want a bit of texture.
Form the Meatballs. Use your hands or a small scoop to form the mixture into 1.5-inch balls.
Cook the Meatballs. You can either: Bake at 400°F for 25–30 minutes, flipping halfway, or Pan-fry in a bit of olive oil over medium heat until browned on all sides.
Serve. Serve with marinara sauce and pasta, on hoagie rolls with melted vegan cheese, or as cocktail meatballs with toothpicks.