Cook the macaroni following pack instructions, adding the peas for the final 3 mins. Drain, reserving a mug of cooking water, then tip the macaroni and peas back into the pan.
Heat grill to medium. Put the pan back on the heat and tip in the mascarpone, mustard and most of cheese. Mix well and add enough of the reserved cooking water to make a creamy sauce that coats the pasta without it looking dry. Stir in the ham and season to taste.
Tip into an ovenproof dish and scatter over the breadcrumbs and remaining cheese, then grill for 5 mins until golden.
