2) Remove the pan carefully from the oven – there will be hot grease in the pan! Transfer the bacon to a plate lined with kitchen paper and crumble when it is cool enough to handle.
3) Drizzle oil into a large pot of boiling salted water. Add the macaroni and cook according to the directions on the package until al dente, 6 to 8 minutes. Drain well.
4) Meanwhile, heat the milk in a small saucepan, but don't boil it. Melt the butter in a medium pot and add the flour. Cook over low heat for 2 minutes, stirring with a whisk. While whisking, add the hot milk and cook for a minute or 2 more, until thickened and smooth.
5) Off the heat, add the Gruyere, Cheddar, blue cheese, 1 tsp salt, pepper and nutmeg. Add the cooked macaroni and crumbled bacon and stir well. Pour into 2 individual size gratin dishes.
6) Place the bread slices in a food processor fitted with a steel blade and pulse to get coarse crumbs. Add the basil and pulse to combine. Sprinkle the breadcrumb mixture over the top of the pasta. Bake for 35 to 40 minutes, or until the sauce is bubbly and the macaroni is browned on top.