Drizzle the olive oil in a large skillet over medium-high heat. Saute the onion and mushrooms until tender, about 8 to 10 minutes. The mushrooms will release liquid as they cook, but this will evaporate as you cook them. Set the pan aside to cool briefly.
Preheat the oven to 400ºF and line a large baking sheet with parchment paper. In the bowl of a 12-cup food processor, combine the lentils, oats, parmesan, egg, oregano, garlic powder, salt, and pepper. Add the cooked mushrooms and onions when they have cooled down a bit. (You don't want to scramble the egg or melt your food processor with piping hot veggies.)
Secure the lid and pulse the mixture several times. It should look relatively uniform, but not pureed. The mixture should stick together well when pressed between your fingers and you should see small chunks of onion and oats throughout. (See photos for reference.)
Use a heaping tablespoon or cookie scoop to scoop the mixture and roll it between your hands to form a ball. Set the ball down on the lined baking sheet, then repeat with the remaining mixture. You should get approximately 24 balls from this recipe.
Bake at 400ºF for 15 minutes, or until the vegetarian meatballs smell fragrant and look dry around the edges. Cook for up to 5 minutes more, if needed. The balls will feel tender when you remove them from the oven, but will firm up more as they cool.
Simmer these in your favorite marninara sauce and serve over pasta for a comforting vegetarian meal. Leftover meatballs can be stored in an airtight container in the fridge for up to 5 days.
Notes
Nutrition information is for 6 meatballs, assuming you get 24 in total. This information is automatically calculated and is just an estimate, not a guarantee.Substitution Ideas: I initially tested this recipe with cooked brown rice instead of oats, but rice is a more challenging ingredient to always have on hand. Feel free to use rice instead of oats if you prefer.Vegan Note: You may be able to use vegan Parmesan and a flax egg to make these vegan, but I haven't tested that yet. You could also use my vegan meatloaf recipe and turn them into balls for a faster cooking time.