Cook the macaroni in boiling water until al dente.
Make a béchamel sauce by melting the butter in a saucepan, then whisking in the flour to make a roux. Gradually add the warm milk, whisking continuously, until the sauce thickens.
Add the mustard, Gruyère, Red Leicester, and Parmesan to the béchamel sauce. Stir until the cheeses have melted and the sauce is smooth.
Drain the cooked macaroni and add it to the cheese sauce, stirring to coat the pasta.
Make croutons by cubing the bread and frying in a pan with the chicken skin until golden brown.
Add a dash of sherry vinegar and Dijon mustard to the mac and cheese.
Tear the mozzarella and stir it into the mac and cheese.
Transfer the mac and cheese to a baking dish and top with the croutons and chicken skin.
Broil or bake until the top is golden brown and bubbly.
