Food Network shares how to make a roux, which is the base to thicken sauces and soups. It’s made from equal parts butter or oil and flour. For a light roux, melt butter over medium-low heat, add flour and stir constantly with a wooden spoon in a figure eight for even cooking. In three to five minutes the light roux will be slightly puffed. It can be used in white sauces like white pepper gravy or bechamel sauce. A brown roux is perfect for gravy and will need to cook for six to seven minutes. Dark roux is great for Cajun and Creole recipes and needs to cook for eight to 15 minutes. Keep in mind that the longer a roux cooks, the less thickening power it will have. Always let your roux cool slightly before adding another liquid, like stock or milk, then whisk and simmer to your desired thickness.
Place a baking rack on a sheet pan and arrange the bacon in 1 layer on the baking rack. Bake for 15 to 20 minutes, until the bacon is crisp. Remove the pan carefully from the oven - there will be hot grease in the pan! Transfer the bacon to a plate lined with paper towels and crumble when it is cool enough to handle.
Drizzle oil into a large pot of boiling salted water. Add the macaroni and cook according to the directions on the package, 6 to 8 minutes. Drain well.
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. Off the heat, add the Gruyere, Cheddar, blue cheese, 1 teaspoon salt, pepper, and nutmeg. Add the cooked macaroni and crumbled bacon and stir well. Pour into 2 individual size gratin dishes.
Place the bread slices in a food processor fitted with a steel blade and pulse until you have coarse crumbs. Add the basil and pulse to combine. Sprinkle the bread crumb 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 the top.