Wring the defrosted spinach dry in a clean kitchen towel. Place the turkey in a large bowl and make a well in the center. Add the spinach, all but 3 tablespoons of the onions, all the garlic, the egg, about ¼ cup of the milk, the bread crumbs, Parmesan cheese, salt and pepper. Mix well.
Form into 12 large balls and drizzle with oil. Arrange on a cookie sheet and bake for 20 minutes. (If you are doing OAMC, allow to cool and label and freeze at this point).
While the meatballs are in the oven, heat a small sauce pot over medium heat. Add a drizzle of oil and the butter. When the butter is melted, add the reserved onions and cook for 2 minutes.
Whisk in the flour and cook for 1 minute, then whisk in the remaining milk and the chicken broth. Bring to a boil, and then stir in the shredded cheese. Season the sauce with salt, pepper and nutmeg; turn the heat to the lowest setting and stir until the cheese melts. Keep warm.
Place 3 meatballs on each dinner plate and top with sauce.