Sauté the onions until tender, 5-6 minutes on medium heat.
Stir in the 3 cloves of minced garlic and cook about 1 minute more. Allow to cool to the touch.
In a medium-sized mixing bowl, combine all ingredients up through the mustard, and stir.
Form the meat mixture into balls of about 2 tablespoons each (aiming for 24 meatballs) and place on a baking sheet. (if using beef you don't need to grease it; if using ground turkey, you should).
Bake at 350 F for 20-22 minutes. If baking into one large loaf, place into a loaf pan and bake for 50-60 minutes.
Once you have placed the baking pan into the oven (or about 20 minutes before you expect a large loaf to be done), start the pasta water to boil. Boil the spaghetti according to the package directions (Banza garbanzo pasta shown, which took 12 minutes).
In the meantime, heat the oil or ghee in the same skillet you used for the onions. Saute the garlic until just tender, 2-3 minutes. Stir in the remaining parsley and ½ teaspoon salt. Once the pasta is done to your taste, drain it and place into the skillet. Stir to coat with the garlic-oil-parsley mixture.
Serve the seasoned pasta with meatballs and any condiments your eaters like with meatloaf. It's got lots of flavor on its own! But I will understand if they want to add ketchup!