Method

• Place the macaroni in a large pan of boiling water and cook following the pack instructions. Drain, reserving 75ml of the cooking water. Meanwhile, heat the oil in a pan and soften the leeks for 5 minutes. Transfer to a plate.
• Melt 20g of butter in the pan, then sprinkle over the flour and mix into a firm paste. Cook for a minute then gradually add the milk, constantly stirring then whisking until you have a smooth sauce. Add the cheese, crème fraîche, reserved pasta water, mustard powder and nutmeg. Whisk again until smooth, with a little black pepper.
• Take off the heat and stir through the pasta and leeks. Spoon into a 2-litre baking dish. Preheat the grill to high.
• Melt the remaining butter and stir it into the breadcrumbs. Scatter evenly over the macaroni cheese, lay the tomatoes cut-side up and press them in slightly. Top with the Parmesan and grill for 10 minutes until golden and bubbling. Serve with a green salad.