Preheat the oven to 375°F. Bring a large pot of salted water to a boil. Drizzle oil into the boiling water, add the macaroni, and cook according to package instructions, 6-8 minutes. Drain well.
Meantime, heat the milk in a small saucepan, but do not allow it to boil. Melt 6 tablespoons butter in a large (4-quart) pot and add the flour all at once. Cook over low heat for 2 minutes, stirring with a whisk. Whisk in the hot milk and cook for a minute or two more, until thickened and smooth. Off heat, stir in the Gruyère, Cheddar, 1 tablespoon salt pepper, and nutmeg. Add the cooked macaroni and stir well. Pour into a 3-quart (9 x 13-inch) baking dish.
Melt the remaining 2 tablespoons of butter, toss with the fresh bread crumbs to coat evenly, and sprinkle all over the top of the casserole. Bake for 30-35 minutes, or until the sauce is bubbly and the macaroni is browned on top. This Recipe Appears In What's the Best Mac and Cheese Cheese?
