PREHEAT the oven to 425°F.
CUT the spaghetti squash lengthwise in half (see Notes) and remove the seeds. Drizzle with the olive oil and season with salt and pepper. Place cut side down on a baking sheet and bake for 40 to 50 minutes, until easily pierced with a fork. Let cool slightly. Use a fork to pull the squash pulp into shreds.
COMBINE the ground chicken, egg, nutritional yeast, flour, parsley, garlic, porcini mushroom powder, salt, and pepper in a large mixing bowl. Gently mix with your hands until the ingredients are fully incorporated. Form the mixture into 16 to 18 equal-size meatballs.
HEAT the olive oil in a large saucepan over medium-high heat. Add the meatballs and cook until lightly browned on all sides. Transfer the browned meatballs to a plate. Reserve the pan.
HEAT the olive oil over medium-low heat in the same pan that the meatballs were browned in. Add the onion with a pinch of salt and cook, stirring, until softened and translucent, 3 to5 minutes. Add the garlic and the crushed tomatoes. Cover and cook at a steady simmer for 15 minutes.
ADD the parsley and basil and season with a pinch of salt and pepper to taste. Return the meatballs to the pan. Stir to coat the meatballs, cover the pan, and continue to simmer for an additional 15 minutes, until sauce has thickened and meatballs are cooked through.
SERVE the meatballs and sauce with the spaghetti squash, additional fresh herbs, and a pinch of nutritional yeast.
NOTES: Make sure to purchase ground chicken that has adequate fat content (not extra lean) or the meatballs will be on the drier side. Or for richer meatballs, you can substitute 1 pound ground beef mixed with 1⁄2 pound ground pork for the ground chicken.
You can also try cutting the spaghetti squash in half widthwise—you’ll get longer strands.