How to Make It
Preheat the oven to 450°. In a small saucepan, bring the milk and heavy cream to a simmer. Keep warm over very low heat.
In a medium saucepan, melt the butter. Whisk in the flour and cook over moderate heat until bubbling, 1 to 2 minutes. Add the garlic, thyme, cayenne, nutmeg and white pepper and whisk until the roux is lightly browned, 3 to 5 minutes. Gradually whisk in the milk and cream until the sauce is smooth and bring to a boil. Simmer over moderate heat, whisking, until no floury taste remains, 5 to 7 minutes. Remove from the heat and whisk in the mild cheddar and 1/2 cup of the sharp cheddar. Season the cheese sauce with salt and black pepper.
In a large skillet, heat 1 tablespoon of the oil. Add the panko and toast over moderately high heat, stirring, until lightly browned, 3 minutes. Transfer to a plate. Wipe out the skillet.
Heat the remaining 2 tablespoons of oil in the skillet. Add the andouille, bell pepper and onion and cook over moderate heat until the vegetables are lightly browned, 5 minutes. Stir in the 1/4 cup of sliced scallions and the chopped cilantro and parsley.
In a large pot of salted boiling water, cook the pasta until al dente. Drain well, then return the pasta to the pot. Stir in the cheese sauce and the andouille mixture. Season with hot sauce and salt and black pepper.
Spoon the pasta into four 12-ounce gratin dishes set on a baking sheet. Top with the remaining 1 cup of sharp cheddar and the toasted panko. Bake until piping hot, 15 to 20 minutes. Let stand for 5 minutes. Garnish with scallions, cilantro and red chiles and serve with hot sauce.