Preheat the oven to 400 degrees F (200 degrees C).
Cook the macaroni in a large pan of boiling salted water. It needs to be cooked to just under what you would normally do, as the pasta will be cooked again in the oven. Drain, then return to the pan and set aside.
Fry the pancetta in a medium skillet over gentle heat until it just starts to brown and crisp up, then add the thyme leaves and scallions and cook for an additional 3 minutes. Remove the skillet from the heat and add its contents to the pasta.
For the sauce, put the butter, flour, nutmeg, and mustard in a small pan set over medium heat and cook 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 pan, stirring well. Keep adding the milk mixture, bit by bit, stirring well each time. This will prevent the sauce from going lumpy. The temptation is to put all the milk in at once only to find you are left with clumps of flour floating on the surface. Should this happen, take the pan off the heat and whisk it like crazy. This normally does the trick to eliminate all the lumps! Make sure you get the spoon into the "corners" of the pan, as stray mounds of flour often lurk there. Once the sauce has fully come together, turn up the heat and boil for a minute or two. The sauce will thicken considerably, then 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. Stir everything together and spoon into a shallow casserole dish or 4 individual large ramekins.
Sprinkle the top with the rest of the cheese and the bread crumbs and bake in the oven for about 20 minutes, or until the cheese starts to bubble and the topping goes crumbly and brown. Sprinkle with chopped parsley and serve while hot.