Preheat oven to Gas Mark 4-5, 180°C, 350°F
In a non-stick pan dry fry lean lamb mince for 4-5 minutes with onion, garlic and thyme until browned. Drain off the fat, add stock and season, cook for 2-3 minutes.
Meanwhile blanch together in boiling water the leeks, potato and fennel. Drain well.
Using an ovenproof shallow dish place half the mince mixture on the base of the dish. Top with half the leeks, potato and fennel. Finishing with the top layer of potatoes.
Mix yogurt with cheese, pour over dish. Sprinkle with breadcrumbs, loosely cover with foil and cook for 10 minutes. Remove foil and bake for a further 30 minutes until topping is golden brown.