Preheat the oven to 400°. Wring the spinach dry in a clean kitchen towel. Place the ground turkey in a large bowl and make a well in the middle of the turkey. Add the spinach, half of the onion, the garlic, the egg, 1/4 cup of the milk, the bread crumbs and the Parmigiano-Reggiano cheese and season with salt and pepper. Mix until combined, then form the turkey mixture into 12 large balls, arrange on a nonstick baking sheet and drizzle with EVOO. Roast for 25 minutes.
Meanwhile, heat a small saucepan over medium heat. Add a drizzle of EVOO and the butter to the pan. Melt the butter, add the remaining onion and cook for 2 minutes, then whisk in the flour. Cook for 1 minute, then whisk in the remaining 1 1/2 cups of milk and the broth. Bring the liquid to a boil, then stir in the provolone. Season the sauce to taste with salt, pepper and the nutmeg.
Place 3 turkey meatballs on each of 4 dinner plates and drizzle with the sauce; garnish with the parsley and serve.