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.
Add cooked penne to the skillet. Stir until bechamel coats all the pasta. Spread the penne in an even layer. Top with mozzarella cheese, and add remaining bechamel sauce. Bake for 15-20 minutes or until top is golden brown and bubbly. Let sit for 10 minutes before devouring!
