Step 1

Heat the oven to 400°F: Rub the 9x13-inch baking dish with a little butter.

Step 2

Cook the pasta: Bring a large amount of water to boil in a large pot. Add the pasta and a generous amount of salt. Cook until the pasta is al dente, then drain and set aside.

Step 3

Make the roux: Melt the butter in a saucepan over medium heat. Sprinkle the flour evenly over top and whisk to form a thick, smooth paste without clumps. Cook for a minute or two, until fragrant.

Step 4

Warm the milk: Warm the milk for a few minutes in the microwave or in a separate pot over low heat. Warm just enough so that it's warm to the touch — warm milk mixes more easily into the roux.

Step 5

Whisk the milk into the roux: Slowly begin pouring the warm milk into the roux while whisking. The roux will at first thin out, then seize up into dry-looking clumps, then smooth out to the consistency of mashed potatoes, and then finally settle into a sauce — keep adding the milk slowly while whisking as you move through these stages.

Step 6

Cook the sauce until thickened: Continue cooking the sauce over medium heat until it has noticeably thickened and clings to the back of a spoon, 5 to 8 minutes. Stir slowly while the sauce thickens, being sure to scrape the bottom of the pan. (If your sauce gets a little too thick, you can pour in a little extra milk to thin it out; no need to warm this extra milk.)

Step 7

Add the cheese: Remove the pot from heat, and stir the cheese into the sauce one handful at a time. Stir in the salt and mustard powder (if using).

Step 8

Pour the sauce over the pasta: Transfer the cooked pasta to a big mixing bowl and pour the sauce over the pasta. Stir gently until the pasta is entirely coated. (If the saucepan you used to cook the sauce is big enough, you can mix the pasta and sauce in the pan instead.)

Step 9

Transfer to the baking dish and top with panko crumbs: Scrape the pasta into the baking dish and smooth the top. Sprinkle the panko crumbs evenly over top.

Step 10

Bake until bubbly and golden: Place the casserole in the oven and bake until the pasta is bubbly and the top is golden, 10 to 15 minutes. If the pasta becomes bubbly before the top is golden, you can run the dish under the broiler for a minute or two to crisp the topping.