4 tablespoons unsalted butter, divided
3/4 cup panko (Japanese breadcrumbs)
1/4 cup finely grated Parmesan
2 teaspoons fresh thyme leaves
1 teaspoon kosher salt, divided, plus more
8 ounces cavatappi or other short curly pasta
2 1/2 cups whole milk
1/2 small onion, grated
1 garlic clove, finely grated
2 tablespoons all-purpose flour
4 ounces Fontina cheese, grated (about 1 cup)
4 ounces Gruyère, grated (about 1 cup)
4 ounces sharp white cheddar, grated (about 1 cup)
1/2 teaspoon English mustard powder (such as Colman’s)
Pinch of cayenne pepper
Preheat oven to 350°. Melt 2 Tbsp. butter in a medium skillet over medium heat. Add panko and cook, stirring, until crumbs are golden brown, 6–8 minutes (make sure to get them toasty brown; they won’t darken much during baking). Transfer to a small bowl and toss with Parmesan, thyme leaves, and ¼ tsp. salt.
Cook pasta in a large pot of boiling salted water, stirring occasionally, until al dente (the noodles will continue to cook in the cheese sauce, so take them out a minute or two before you think they’re actually done). Drain pasta; let cool while you make the sauce.
Bring milk to a bare simmer in a small saucepan; keep warm. Melt remaining 2 Tbsp. butter in a medium saucepan over medium-high. Add onion and garlic and cook, stirring, until onions are fragrant and beginning to soften, about 2 minutes. Sprinkle flour over and cook, stirring constantly, until mixture starts to stick to bottom of saucepan, about 1 minute. Add warm milk in a few additions, whisking to combine after each addition.
Bring béchamel sauce to a boil, then reduce heat and simmer, stirring, until sauce is thickened and doesn’t feel grainy when a little bit is rubbed between your fingers, 6–8 minutes (cooking the flour thoroughly at this stage ensures a creamy sauce). Add Fontina, Gruyère, cheddar, mustard powder, cayenne, and ¾ tsp. salt and stir until cheeses are melted and sauce is smooth. Remove from heat and mix in pasta; transfer to a 2-qt. baking dish.
Bake 10 minutes. Top with Parmesan breadcrumbs and bake until sauce is bubbling around the edges, 8–10 minutes longer. Let cool in pan 15 minutes before serving.
From Bon Appetit.