- 1 pound pasta shells
- 4 tablespoons butter
- 1/2 onion, finely chopped
- Kosher salt and freshly ground black pepper
- 1/4 cup all-purpose flour
- 3 cups whole milk, warmed
- 2 teaspoons Dijon mustard
- 2 cups shredded 5-cheese Italian blend
Melt the butter in a large saucepan. Add the onion and saute until tender, about 3 minutes. Season with salt and pepper. Add flour and stir until it looks pasty and reaches a golden blonde color, about 2 minutes.
Slowly whisk in the warm milk, bringing up to a simmer, and allow the mixture to thicken. Stir in the mustard. Let simmer on low heat for 5 minutes, until thick enough to coat the back of a spoon.
Gradually stir in the cheese a handful at a time. Stir in the pasta, making sure it's all covered with the cheese sauce and serve hot.