Prepare béchamel sauce: Melt butter in a pan, stir in flour to create a roux. Gradually add milk until you achieve a smooth consistency. Add a dash of sherry vinegar, salt, and mustard. Adjust the consistency with more milk if needed.
Cook macaroni: Cook macaroni in boiling water until 75% done. Drain, reserving some pasta water.
Combine sauce and pasta: Mix the cooked macaroni into the béchamel sauce, adding a bit of pasta water to adjust consistency.
Add cheeses: Gradually add grated French Gruyère, Red Leicester, and Parmesan to the sauce. Adjust the sauce consistency as needed.
Prepare croutons and chicken skin: Toast chunks of bread in a pan with fat until golden. Bake chicken skin at 160°C until crispy. Break into pieces.
Final touches: Mix in torn mozzarella into the mac and cheese. Top with croutons and chicken skin. Serve immediately.
Bake: Transfer mac and cheese into a baking dish, top with additional cheese if desired, and bake under a preheated grill until bubbly and golden.