1) Make the Simple lamb casserole with onions and new potatoes, but do not add the new potatoes.
2) Once the lamb casserole is cooked, remove from the oven and leave to cool for 1 hour. Carefully transfer the lamb to a large ovenproof frying pan, making sure the pieces are not on top of each other. Preheat the oven to 200C/Gas 6.
3) Place the new potatoes in a pan, cover with cold water, season with salt and cook for 10 to 15 minutes until soft.
4) Bring a pan of salted water to the boil and cook the peas for 2 minutes. Remove with a slotted spoon and plunge into iced water. Cook the baby leeks in the boiling water for 3 to 4 minutes, then remove and plunge into iced water. Cook the baby carrots for 4 to 5 minutes, then remove and plunge into iced water.
5) Strain the casserole liquid through a fine sieve over a saucepan. Discard the vegetables. Ladle some of the liquid over the meat, bring to a boil and place in the oven for 10 to 15 minutes. Occasionally open the oven to spoon the juices over the meat.
6) Heat the remaining liquid in a pan and reduce by a third. Place the potatoes and vegetables into a pan, add 100ml of water, the butter and seasoning, and warm through.
Once the lamb is shiny and glazed, remove from the oven. Place four or five pieces into a bowl, spoon over some of the vegetables, pour over the sauce and finish with scattered mint leaves.