Preheat the oven to 375°F (190°C).
Cook the penne pasta according to the package instructions. Drain and set aside.
In a large saucepan over medium heat, melt the butter. Add the flour and whisk continuously to form a roux. Cook for 1-2 minutes until it turns golden brown.
Gradually whisk in the milk to the roux, ensuring there are no lumps. Continue whisking until the mixture thickens.
Add the Dijon mustard, garlic powder, salt, black pepper, and nutmeg (if using). Stir to combine.
Reduce the heat to low, then add the shredded cheddar, mozzarella, and grated Parmesan cheeses. Stir until the cheeses melt and the sauce is smooth and creamy.
Add the cooked penne pasta to the cheese sauce, stirring to coat the pasta evenly.
Transfer the cheesy pasta mixture to a greased baking dish.
Sprinkle additional shredded cheddar and mozzarella on top for a cheesy crust.
Bake in the preheated oven for 20-25 minutes or until the top is golden and bubbly.
Remove from the oven and let it rest for a few minutes before serving.
Garnish with chopped fresh parsley if desired.
