Bring a large pot of salted water to a boil over high heat. Add pasta and cook according to the packaging's instructions. Once cooked, drain into a colander, and let cool.

In the same pot, add butter and melt over a low heat. Once melted, whisk in the flour. Let the flour and butter (roux) cook and bubble for two minutes.

Slowly, whisk in the milk. Add the garlic, salt, pepper, and mustard. Increase the heat to medium/high. Do not let the milk boil. Whisk every so often until the sauce has thickened up a bit, about 10 minutes.

Add in the cheese. Turn the flame off. Whisk the cheese into the sauce until it is completely melted. Add the shells back to the pot. Stir to combine.

Stir in the chopped sundried tomatoes and basil. Taste and add more salt and pepper if needed.

Let the shells and cheese sit for five minutes before serving so the sauce has time to thicken up a bit.