Preheat the broiler and position a rack 6 inches from the heat. In a deep, medium skillet, cook the bacon over moderately high heat until browned and crisp, about 6 minutes. Drain the bacon on paper towels and discard all but 1 tablespoon of the fat in the skillet.
Add the onion to the skillet and cook over moderate heat, stirring, until softened and lightly browned, about 6 minutes. Add the flour and cook, stirring, for 1 minute. Add the milk and whisk until the sauce is thickened and bubbling, about 3 minutes. Turn off the heat and stir in the Brie until completely melted.
Meanwhile, in a large pot of boiling salted water, cook the fusilli until al dente; drain well.
Add the fusilli to the sauce in the skillet and toss. Add the bacon and season with salt and pepper. Transfer the pasta to an 8-inch baking dish and sprinkle the Parmigiano-Reggiano on top. Broil for 5 minutes, until the cheese is golden, shifting the pan for even browning.