For the turkey meatballs: Add the oats to a food processor and pulse a few times. The consistency should resemble breadcrumbs. Add the oats and milk to a small bowl and set aside for 5 minutes.
Add the onions, spinach, Parmesan, salt, pepper, garlic and egg to a large bowl and mix well. Add the oat mixture and combine. Lastly, add the turkey and gently fold into the wet mixture. Keep the mixture light by not over-mixing. Form into bite-size balls using a small cookie scoop for uniform meatballs.
Heat 4 tablespoons of the oil in a large Dutch oven over medium heat. Add half of the meatballs and brown on all sides, 5 to 7 minutes. Remove and repeat with the second batch, adding more oil if needed. Set the meatballs aside but reserve the Dutch oven for the sauce.
For the quick marinara sauce: Add the tomatoes, tomato paste, oregano, salt, pepper and garlic to the Dutch oven and stir. Bring to a boil. Reduce the heat, cover and let simmer for about 10 minutes. Gently add the browned meatballs to the sauce. Do not overcrowd. Simmer until the meatballs are cooked thoroughly, another 10 to 15 minutes. If making the optional spicy version, add the red wine and chile flakes in the last 2 minutes of cooking.
Bring a large pot of water to a boil. Season with a generous pinch of salt. Add the spaghetti and cook until al dente, about 8 minutes.
Combine some of the sauce with the cooked spaghetti. Plate the spaghetti and meatballs and add the remaining sauce over everything. Garnish with parsley, basil and grated Parmesan. Serve hot.