- Get a large pot of water boiling for your noodles. When the water boils, cook the macaroni for about 2 minutes under the recommended cooking time. Drain pasta in a colander and rinse with cold water.
- Meanwhile, warm milk in a medium saucepan or in the microwave.
- Melt 2 1/2 tablespoons butter in a high-sided skillet over medium heat. When the butter bubbles, add flour. (Make sure you use a pan that isn't very wide. The flour will seize up if the butter isn't in a smaller area.) Cook, stirring, 1 minute.
- Slowly pour hot milk into flour-butter mixture while whisking. Continue cooking, whisking constantly, until the mixture bubbles and becomes thick.
- Remove the pan from the heat. Stir in salt, nutmeg, black pepper, 1 1/2 cups cheddar, and 1/2 cup pecorino Romano. Mix drained noodles and cheese sauce in a 4-quart baking dish.
- Sprinkle remaining 3/4 cup cheddar and 1/4 cup pecorino Romano over the noodles and sauce. Add chopped bacon and scatter breadcrumbs or potato chips over the top.
- Take the remaining 1/2 tablespoon of butter and drop little pats of butter across the top of the breadcrumbs. (You don't need extra butter if you use potato chips.) Bake at 400 degrees until browned on top, about 30 minutes. Cool for 5 minutes and then serve.