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Boil the potatoes in a saucepan of boiling water for 15 mins until par-boiled, then drain and leave to steam dry and cool.
Heat oven to 200C/180C fan/gas 6. Heat the oil in a large frying pan and fry the onion for 10 mins or until starting to soften. Add the carrot and cook for 5 mins until tender. Tip the carrot and onion mixture into a large 2-litre ovenproof dish. Put the pan back over high heat. Fry the lamb meatballs, turning, until golden on the outside. Put the meatballs into the ovenproof dish. Pour over the gravy and tuck in the bay leaves and thyme sprigs.
Thinly slice the cooled potatoes and arrange over the top of the meatballs. Brush the potato with 1-2 tsp oil and add a good grinding of black pepper. Bake in the oven for 20-25 mins until the potatoes are tender and starting to become crisp at the edges, and the gravy is bubbling.