3 tablespoons unsalted butter
3 poblano peppers
1 pound dried conchiglie or penne pasta
4 cups hot Béchamel sauce (recipe follows)
16 ounces sharp white cheddar, grated
8 ounces Parmigiano-Reggiano or Asiago, grated
1/2 teaspoon cayenne pepper, or to taste
1/2 teaspoon paprika
1/2 teaspoon dry mustard
1/2 cup breadcrumbs
6 tablespoons (3/4 stick) unsalted butter
1/3 cup unbleached all-purpose flour
4 cups whole milk, heated until steaming
1/2 teaspoon sea salt
1/2 teaspoon grated nutmeg
Preheat oven to 400°F. Butter a 9 x 13-inch baking dish. Lay poblanos on their sides on a gas burner and turn flame on high. Roast chiles, turning with tongs, until skins are evenly charred, about 10 minutes. Steam until cool in a closed paper bag, about 15 minutes. Under cold running water, rub off burned peel. Then, discard stems, core and devein chiles (scraping away all seeds), and cut into long, thin strips.
To make béchamel, melt 6 tablespoons butter over low heat. Whisk in flour and cook, whisking often, without allowing flour to brown, about 4 minutes. Remove pan from heat and whisk in hot milk. Add salt and nutmeg. Return sauce to medium heat and bring to simmer. Continue to cook, stirring with a wooden spoon, until sauce has thickened to texture of melted ice cream, about 10 minutes.
Meanwhile, bring a large pot of salted water to a boil. Add pasta and cook according to instructions on package, until just tender. Drain pasta and transfer to large bowl, and toss with butter. Stir in cheddar cheese and all but 1 cup of the grated Parmigiano-Reggiano into the hot béchamel. Add cayenne, paprika, and mustard. Heat, stirring, until cheeses melt and sauce is smooth.
Pour the sauce over the pasta in the bowl. Add the chiles and stir. Spoon into the prepared dish. In a small bowl, combine reserved cup of grated cheese and the breadcrumbs. Sprinkle breadcrumbs evenly over pasta in the dish. Bake until pasta is heated through, sauce is bubbling, and top is browned, about 20 minutes. Remove from oven and let stand on a rack for 5 minutes. Serve.