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Heat oven to 200C/180C fan/gas 6. Rub the peeled garlic clove around the inside of a baking dish (ours was 18cm x 25cm). Mix the cornflour with 1 tbsp cream in a bowl, then whisk in the remaining cream, crème fraîche and mustard, and season well. Add the beans, lemon zest and parmesan, and stir until well coated in the cream mixture. Pour into the dish, then put on the middle shelf of the oven while you cook the lamb.
Season the lamb all over. Heat the oil in a frying pan and sear the fatty side of the lamb for 1-2 mins until well browned. Add the herbs and garlic to the hot oil and sizzle for 30 seconds.
Sit the lamb on top of the beans along with the garlic, herbs and slices of lemon. Return to the oven for 15-20 mins, depending on how well cooked you like your lamb. Serve with a bowl of seasonal greens on the side.