In a large stockpot, melt butter over meium heat. Add onion and saute until softened, about 4 minutes. Add garlic and saute until aroma is released, about 30 seconds. Add spinach and broth and simmer 5 minutes to allow flavors to blend. Puree soup in blender and batches and return to pot. Add cream and cook until heated through. Finish with nutmeg and season to taste with salt and pepper.