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Heat the oil in a saucepan. Add the carrot, onion, celery, thyme and bay leaf, and cook over a medium heat for 10 mins to soften. Add the lentils and fry gently for 1-2 mins. Add the stock, bring to the boil and cook, uncovered, for 20-25 mins until the lentils are cooked through and most of the stock has been absorbed.
Meanwhile, cook the lamb. Heat oven to 200C/180C fan/gas 6. Place the lamb in a roasting tin, season well and roast for 15-20 mins for rare to medium, 25 mins for well done. Leave to rest for 5 mins before carving into chops.
While the lamb is resting, make the sauce. Put the Jack-by-the-hedge in a food processor and blitz while slowly trickling in the oil until it forms a pesto consistency. Add the lemon juice to taste, then season.
Stir the butter, mustard and any juices from the lamb into the lentils and season to taste. Divide the lentils between warmed plates, top with the chops and drizzle with the sauce.