Preheat the oven to 400°F. Butter a shallow 4-quart baking dish (or use 2 smaller baking dishes, or one smaller baking dish and some individual ramekins; see the Fork in the Road, opposite page).

Bring a large pot of water to a boil, salt it generously, and let the water return to a boil.

Meanwhile, make the panko topping: Melt the butter in a small saucepan over low heat or place it in a medium-size microwave-safe dish and heat it in a microwave oven until melted, 15 seconds. Add the panko and the Parmesan and stir until well combined. Set the panko topping aside.

Make the pasta and sauce: Melt the butter in a large heavy saucepan over medium heat. Whisk in the flour and red pepper flakes, if using. Cook, stirring, until the flour is blond in color, about 4 minutes. Gradually whisk in the milk. Increase the heat to medium-high and let come to a simmer, whisking frequently. Reduce the heat to medium-low and let the sauce simmer until it starts to thicken, about 5 minutes. Add the cream, grated cheese, Parmesan, mustard, salt, and black pepper, stirring until everything is smooth. Taste for seasoning, adding more salt and/or black pepper as necessary.

Add the pasta to the boiling water and cook it until barely al dente (follow the package directions but stop a minute or two before the pasta is completely tender). Set aside 1 cup of the pasta cooking water, then drain the pasta.

Whisk the reserved pasta cooking water into the cheese sauce, combining it thoroughly. Add the pasta to the cheese sauce and stir to combine. Spoon the pasta mixture into the prepared baking dish. There will appear to be a lot of sauce. Some of it will be absorbed into the pasta as it cooks, and in my book saucy is better than dry.

You can continue with Step 8 or see the Fork in the Road suggestion on preparing the mac and cheese for kids below.

Sprinkle the panko topping evenly over the pasta and bake it until golden and bubbling, 30 to 40 minutes. Let the pasta sit for a few minutes before serving.