1 Pound(s) pasta, elbow
5 Tablespoon(s) unsalted butter
6 Tablespoon(s) flour
1 1/2 Teaspoon(s) mustard, dry
1/4 Teaspoon(s) cayenne pepper
5 Cup(s) milk
8 Ounce(s) shredded cheese, monterey jack
8 Ounce(s) shredded sharp cheese, cheddar
1 Teaspoon(s) salt
Adjust oven rack to lower-middle position and heat broiler. Bring 4 quarts water to boil in Dutch oven over high heat. Add macaroni and 1 tablespoon salt; cook until pasta is tender. Drain pasta and set aside in colander.
3. In now-empty Dutch oven, heat butter over medium-high heat until foaming. Add flour, mustard, and cayenne (if using) and whisk well to combine. Continue whisking until mixture becomes fragrant and deepens in color, about 1 minute. Gradually whisk in milk; bring mixture to boil, whisking constantly (mixture must reach full boil to fully thicken). Reduce heat to medium and simmer, whisking occasionally, until thickened to consistency of heavy cream, about 5 minutes. Off heat, whisk in cheeses and 1 teaspoon salt until cheeses are fully melted. Add pasta and cook over medium-low heat, stirring constantly, until mixture is steaming and heated through, about 6 minutes.
4. Transfer mixture to broiler-safe 9-by 13-inch baking dish and sprinkle evenly with bread crumbs. Broil until crumbs are deep golden brown, 3 to 5 minutes, rotating pan if necessary for even browning. Cool about 5 minutes, then serve.