Preheat your oven to 350°F. Spray a baking sheet with non-stick cooking spray.
In a large bowl, add the lentils, brown rice, onion, oats, cheese, flax egg, tomato sauce, and seasonings. Mix with your hands until well-combined.
Form the mixture into golf ball-sized meatballs and arrange them on the prepared baking sheet.
Bake for 25 minutes, taking them out halfway through and turning them over.
Make the sauce during the second half of baking. Add all the sauce ingredients to a large skillet. Whisk together and bring to a boil.
Once the sauce starts to boil and thicken up, you can gently add in the meatballs.
Spoon the sauce over the meatballs, cover, and cook for 10 minutes. Gently stir the sauce and meatballs every so often to keep them from sticking to the bottom of the pan.
Once the sauce has thickened, you're done.