Mix all of the meatball ingredients (except ground turkey). Add the meat and incorporate into the ingredients with your hands. Roll into 12 (1-1/2-inch) meatballs and place on a nonstick or parchment-lined baking sheet. Bake for 10 minutes. Turn the meatballs over and bake an additional 10 minutes, or until cooked through.
Meanwhile, pour marinara sauce and crushed red pepper flakes into a saucepan. Cook on the stovetop over low heat until simmering. Add cooked meatballs. Garnish with parsley and minced onion, if using. Enjoy!