Preheat the oven to 350F
Use spray oil to coat a baking dish (a 3-quart size is fine)
Cook the rotini until it’s nearly al dente. Strain and rinse with cold water (this keeps the macaroni from continuing to cook. You don’t want limp noodles).
Toss the pasta with olive oil and set aside
To make your sauce, begin by melting the butter.
Slowly add the flour over medium heat until it’s golden and bubbly. This is the foundation for your roux.
Whisk in the milk until the mixture is smooth.
Stir in the cheese, and continue stirring until everything is well-incorporated
Add your pasta and cook for 30 minutes.