Halve any larger potatoes, then cook in a pan of boiling salted water for 15 minutes, or until tender, adding the peas to the party for the last 3 minutes. Meanwhile, rub the lamb all over with 1 teaspoon of olive oil, and a pinch of sea salt and black pepper. Starting fat side down, sear the lamb in a non-stick frying pan on a medium-high heat for 10 minutes, turning regularly until gnarly all over but blushing in the middle, or use your instincts to cook to your liking. Remove to a plate to rest, then, with the frying pan on a low heat, make a liquor by stirring in a splash of water and a little red wine vinegar to pick up all those nice sticky bits, leaving it to tick away slowly until needed. Drain the potatoes and peas, tip into the liquor pan, pick over most of the basil, add the pesto and toss it all well. Serve with the lamb, sliced thinly, then pour over the resting juices. Pick over the remaining basil leaves, and tuck in.