Preheat Oven: Heat the oven to 400°F (200°C).
Make the Roux: Melt butter over medium heat, then add flour, salt, pepper, and mustard.
Cook the Base: Whisk and cook for 30 seconds until smooth.
Add Milk Gradually: Pour in milk slowly, stirring constantly until smooth.
Melt the Cheese: Add all cheeses and whisk until fully melted.
Combine Pasta: Stir in the cooked rotini until well coated.
Bake: Transfer to a baking dish, cover, and bake for 20 minutes.
Serve: Let cool slightly before serving. Enjoy!