Cook the bacon until crisp. Set on paper towels to drain. When it's cool enough to touch, crumble it.
Cook farfalle almost to al dente according to package directions, then drain and reserve.
Preheat oven to broil and coat an 8x8-inch baking dish with nonstick cooking spray.
While pasta is cooking, make the cheese sauce. In a saucepan over medium heat, melt the butter. Add the flour and cook, stirring, for 1 minute. Slowly add the milk, then salt, pepper, and nutmeg. Cook, stirring, for about 5 minutes to thicken. Remove from heat.
Add cheese to sauce and stir to melt. Stir in the horseradish and cooked bacon.
In a mixing bowl, toss sauce with pasta. Transfer to prepared baking dish, sprinkle with parsley, 1/2 cup cheese, and scallions, and place under broiler. Broil until the pasta is golden brown, about 4 minutes.