Preheat oven to 180°C/350°F/Gas Mark 4. Heat an ovenproof frying pan on the stove. Add butter and as soon as it has melted add the lamb neck fillets. Brown on all sides then place the pan in the oven and roast 5-7 minutes or until fillets are still pink in the middle. Rest meat in a warm place for 5 minutes before serving in ½ cm (¼ in) slices with the salsa on top.
To make the salsa, place peas, spring onions, mint, olive oil, vinegar and salt in a food processor and pulse for 3 seconds. Scrape down the sides and blitz again briefly – the salsa should be a little chunky. Tip into a bowl and taste for seasoning.