Preheat oven to 400 degrees F. In a 12" cast iron skillet over medium heat, melt butter. Add flour and whisk for 2-3 minutes or until mixture is a light golden color. Add milk. Whisk until smooth. Once bechamel has come to a boil, reduce heat to medium-low and let simmer for 2-3 minutes. Remove from heat. Add salt, pepper, garlic, and nutmeg. Set aside. Remove half of the bechamel in a separate bowl.