Grease a 2-qt. baking dish with 1 tablespoon melted butter and set aside. Bring a 6-qt. pot of salted water to a boil, add the pasta, and cook until al dente, about 8 minutes. Drain the pasta, rinse, and set aside. Pulse the bread in a food processor until finely ground, mix with 4 tablespoons butter, and set aside.

​Heat the oven to 400ºF. Melt the remaining butter in a 6-qt. pot over medium heat. Add the thyme and onions and cook until soft, about 6 minutes. Whisk in the flour and cook for about 3 minutes. Whisk in the milk and cream. Increase the heat to medium-high and cook, whisking, until thick, 10-12 minutes. Whisk in the cheddar, ham, blue cheese, parsley, hot sauce, scallions, pepper, and nutmeg. Taste and season with salt as needed. Stir in the pasta, transfer to the prepared baking dish, and sprinkle with the bread crumbs. Bake until golden brown and bubbly, 30-40 minutes.

​Cooking note: There are a few tried-and-true secrets to making macaroni and cheese with the perfect creamy texture. You need a smooth sauce as a base—in this case, it's a velvety béchamel made of butter, flour, milk, and cream. And you need to make sure the cheese melts evenly and becomes one with that sauce. Firm cheeses like cheddar, Comté, fontina, and Gruyère are reliable choices. Let the cheese come to room temperature before you use it, shred it as finely as possible, and heat it gently with the sauce; a blast of high heat can cause proteins in the cheese to separate from the fats, resulting in a macaroni and cheese that's both grainy and oily.