Bring a large pot of water to a boil, salt it, add the pasta and cook until just shy of al dente.
Heat the oven to 350 degrees F. Bake the bacon on a broiler pan until crisp, 25 to 30 minutes (I just use the ready cooked bacon and microwave it until it is crispy). Chop the bacon Preheat the broiler.
Meanwhile, in a medium saucepan, melt the butter over medium heat. Whisk in the flour for 1 minute, then wisk in the milk; season with salt, pepper and nutmeg. Cook until reduced, about 5 minutes. Stir in 2 cups of cheese until melted, then remove the sauce from the heat and stir in the horseradish and teh bacon.
In a casserole dish, combine the pasta with the sauce and top with parsley, scallions and remaining 1/2 cup cheese Broil until browned, 3 to 4 minutes. Serve with a few pickles.