Prepare the oats by pulsing whole oats in a food processor until they resemble fine breadcrumbs, or use quick oats directly.

Finely chop the fresh mushrooms, leaving some larger pieces, and set aside.

In a large mixing bowl, combine the minced mushrooms, cooked brown rice or quinoa, chopped onion, minced garlic, prepared oats, cornstarch, salt, and pepper. Mix thoroughly.

Shape the mixture into small meatballs, about 1-1.5 inches in diameter.

Cook the meatballs in a skillet with olive oil over medium heat for 3-5 minutes on each side until golden brown.

In a separate saucepan, heat olive oil and sauté minced garlic until fragrant. Add tomatoes and simmer for 8-10 minutes.

Gently add the browned meatballs into the sauce and simmer together for a few minutes.