Preheat the oven to 350 degrees F. Lightly spray a baking sheet with cooking oil and set aside.
In a large skillet, heat oil over medium heat. Add onions and mushrooms and cook until onions are almost translucent and the mushrooms have softened, about 5 minutes. Add seasonings and cook 1 minute more.
While mixture is cooking, add sunflower seeds to a food processor and pulse until coarse. Add onion-mushroom mixture along with half of lentils and pulse until mixture is coming together, 10 – 15 pulses. Transfer to a bowl.
Add remaining lentils, quinoa, chickpea flour and nutritional yeast (if using) and stir together to form a dough. Season with salt and pepper.
Shape dough into golf ball-sized meatballs and place on baking sheet. Repeat until no dough remains.
Bake on center rack for 30 minutes, flipping halfway through, until balls are browned and crispy on the outside. Remove and let sit for 5 – 10 minutes.
While balls are resting, spiralize the zucchini (using this spiralizer). Cut into spaghetti sized noodles and place in serving bowls.
Heat sauce over medium-low heat until bubbling. Pour over noodles and top with three meatballs.
Sprinkle with additional red pepper flakes and nutritional yeast if desired.