Grease a 2-qt. baking dish with 1 tbsp. 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 tbsp. butter, and set aside.

Heat the oven to 400°F. 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.