Wring defrosted spinach dry in a clean kitchen towel. Place turkey in a bowl and make a well in the middle of it. Add the spinach, all but 3 tablespoons of the onion, all of the garlic, 1 large egg, about 1/4 cup milk, bread crumbs, grated Parmesan, salt and pepper. Mix well. Form into 12 large balls and drizzle with extra-virgin olive oil. Arrange on a nonstick cookie sheet and roast 20 minutes, or until cooked through.
While balls are in the oven, heat a small sauce pot over medium heat. Add a drizzle of extra-virgin olive oil and 2 tablespoons butter. Melt butter, add remaining finely chopped onion and cook 2 minutes then whisk in flour. Cook flour 1 minute, then whisk in 1 1/2 cups milk and 1 cup stock. Bring liquid up to a boil then stir in shredded provolone or blended Italian cheeses. Season the sauce with salt, pepper and nutmeg, turn heat to lowest setting.
Place 3 balls on dinner plates and top with sauce, garnish with parsley.