Preheat the oven to 400 degrees F. Cook the pasta in a large pan of boiling salted water. It needs to be cooked to just less than al dente, as the pasta will be cooked again in the oven. Reserve 1 cup cooking water, then drain the pasta, return it to the pan and set aside.
Fry the pancetta in a medium skillet over medium heat until it just starts to brown and crisp up, then add the thyme and scallions and cook for an additional 3 minutes. Add the skillet's contents to the pasta.
For the sauce, put the butter, flour, nutmeg and mustard powder in a small saucepan set over medium heat and cook, stirring, until the butter has melted. Mix the milk and cream together in a pitcher and add a little to the flour and butter in the saucepan, stirring well. Keep adding the milk mixture bit by bit, stirring well each time (be sure to get into the "corners" of the pan, as flour often lurks there). Once the sauce has fully come together, turn up the heat and boil for a minute or two. The sauce will thicken considerably. Remove the pan from the heat.
Add two-thirds of both of the cheeses to the sauce while it is still hot and combine well. (It may be a bit lumpy, but that is fine.) Season to taste with salt and pepper and add to the pasta mix. If the cheese sauce thickens too much, add some of the pasta water. Stir everything together and spoon into 4 large ramekins or a shallow 3-quart casserole dish.
Sprinkle the top with the rest of the cheese and the breadcrumbs and bake in the oven for about 20 minutes, or until the cheese starts to bubble and the topping goes brown. Sprinkle with chopped parsley and serve.