Bring a large saucepan of salted water to the boil and cook the macaroni according to the instructions on the packet.
In a medium heavy based saucepan, melt the butter. Once melted, add the flour and cook out the paste for 2 minutes. Add the milk slowly, stirring well after each addition to ensure you have a lump free sauce. Once all the milk has been incorporated, bring the mixture to the boil for 1 minute. Add the mustard and cook for a further minute. Turn off the heat, add the cheese and stir well until fully melted and incorporated. Season to taste. Pour the cheese sauce over the macaroni and mix well to combine before tipping into a baking dish.
Put the sourdough into a food processor and pulse until there’s a nice mixture of fine crumbs, and slightly chunkier pieces remaining. Add the pancetta pieces to a large frying pan over a medium heat until crispy and golden. Remove with a slotted spoon and set aside. Tip the crumbs into the pan and cook for 3-4 minutes in the pancetta fat until lightly golden and starting to crisp. Sprinkle the crumbs over the macaroni, followed by the pancetta, then the cheeses. Cook under the grill for 5 minutes or until toasted and bubbling.