4 tablespoons unsalted butter, plus more for dish
3 cups milk
1 jalapeno pepper, seeded (if desired) and minced
1/4 cup all-purpose flour
1 teaspoon kosher salt, plus more for pasta water
1/4 teaspoon freshly ground black pepper
Generous pinch freshly grated nutmeg and ground cayenne pepper
2 cups (about 8 ounces) grated sharp white cheddar
2 cups (about 8 ounces) grated Monterey Jack cheese
1 pound elbow macaroni (or your favorite shape)
Vegetable oil, for frying
3 corn tortillas, cut into thin strips, plus more for serving
Sour cream, sliced avocado and pico de gallo for serving
1. Heat oven to 375 degrees. Butter a 2-quart casserole dish; set aside. Heat milk in a saucepan set over medium heat. Do not boil. Melt butter in a high-sided skillet over medium heat. When butter bubbles, whisk in jalapeno and flour. Cook, whisking constantly, for 1 minute. While whisking, slowly pour hot milk into flour mixture. Continue cooking, whisking constantly, until the mixture bubbles and thickens. Remove the pan from the heat and stir in salt, pepper, nutmeg, cayenne, 1 1/2 cups cheddar and 1 1/2 cups Monterey Jack; set aside.
2. Bring a large saucepan of salted water to a boil. Add pasta and cook 2 minutes less than manufacturer's directions, until outside of pasta is cooked but inside is still underdone. (The pasta will continue to cook in the oven.) Drain pasta, rinse under cold running water and drain well. Stir pasta into the reserved cheese sauce.
3. Pour the mixture into the prepared casserole dish. Sprinkle remaining 1/2 cup cheddar and 1/2 cup Monterey Jack over the top. Bake until browned on top, about 30 minutes. Transfer dish to a wire rack to cool for 5 minutes.
4. Meanwhile, heat 1/4-inch vegetable oil in a skillet over medium-high heat. Add tortilla strips and fry, stirring gently to ensure even cooking, until golden. Transfer strips to paper towels to drain. Serve pasta with tortillas, sour cream, avocado and pico de gallo.
Serves 6 to 8