Preheat the grill to hot. Bring a large pan of water to a rolling boil and cook the macaroni according to pack instructions. Drain and set aside.
Melt the butter in a large saucepan, add the flour and cook for 1 minute. Gradually, add the milk a splash at a time, stirring continuously to avoid lumps until a smooth sauce has formed. Stir in the mustard and ¾ of the cheese. Then add the cooked macaroni and stir until coated.
Tip into a 2-litre ovenproof dish. Scatter with the remaining cheese followed by the garlic & rosemary crust, drizzle with olive oil and pop under the grill for 5 minutes until golden, bubbly and crisp on top. Serve with dressed salad leaves.
