Melt the butter in a small pan or in the microwave. Place in a food processor with the pistachio nuts and bread crumbs and blitz until you have achieved fine crumbs. Season to taste.
Place the racks of lamb on a chopping board and, using a pastry brush, spread the mustard thickly over the fat side of each rack. Cover with the pistachio nut crust using your hands press it down, moulding it over the lamb. Arrange the lamb, coated side up, on a baking sheet and chill for at least 30 minutes, or up to two hours, to allow the crust to "set". Remove from the fridge 10 minutes before cooking.
Preheat the oven to 200C/400F/Gas 6. Place the racks of lamb in a small roasting tin and roast for 20 minutes per 450 grams plus 20 minutes for pink lamb, or longer if you prefer your meat well done.
Remove the lamb from the oven and set aside in a warm place to rest for 10 to15 minutes, then carefully carve into chops and serve on warmed plates.
Serve with new potatoes and spring vegetables or a green salad.
Original recipe taken from Ainsley Harriott’s “Just Five Ingredients” published by BBC Books.