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Heat oven to 180C/160C fan/gas 4. Heat a small roasting tin, and coat the lamb with 1 tbsp of the oil and some sea salt. Place the lamb, skin-side down, in the tin until the fat has turned brown, then sear all over. Transfer to a plate.
Add the onion to the tin and cook for about 10 mins until soft. Stir in the potatoes, thyme, garlic and stock, and press down with a spoon. Put the lamb on top and roast in the oven for 25-30 mins until the potatoes are tender and the lamb cooked to your liking.
Leave to rest under some foil for about 10 mins. Slice the lamb into cutlets and scatter with the reserved thyme sprigs. Serve with a spoonful of the potatoes and some simply cooked asparagus, if you like.