How to Make It
Preheat the oven to 350°. Butter 4 small soufflé dishes or a shallow 1 1/2-quart gratin dish.
Heat the 1 tablespoon of butter in a medium skillet over moderate heat. Add the bacon and cook until crisp and browned, 5 to 8 minutes. Using a slotted spoon, transfer the bacon to a paper towel–lined plate. Set aside.
Add the onion to the skillet and cook until softened, 6 to 8 minutes. Add the flour and cook, stirring until smooth, about 1 minute. Add the milk and cook over moderately low heat, stirring occasionally, until thickened, about 10 minutes. Add all but 1/4 cup of the blue cheese and stir until smooth and melted. Season with salt and pepper, and keep the sauce warm over very low heat.
Meanwhile, in a medium saucepan of boiling well-salted water, cook the pasta until al dente. Drain well and add the pasta to the cheese sauce, stirring well to coat. Stir in the reserved bacon.
Divide the macaroni and cheese between the soufflé dishes or pour into the gratin dish, then top with the remaining blue cheese.
Bake until golden and bubbling, 15 to 20 minutes. Garnish with snipped or finely chopped chives before serving, if desired.