Preheat the oven to 180°C, fan 160°C, gas 4.
Heat half the oil in a large frying pan and cook the lamb in batches for 10 minutes or until just browned. Remove and set aside.
Heat the remaining oil in the pan and fry the onions over a low heat for 10 minutes until golden. Make up 400ml lamb stock using the stock cube. Stir the flour into the pan and then add the stock and Worcestershire sauce. Season to taste with freshly ground black pepper.
Layer up the dish with the lamb, carrots, onions and half the potatoes. Add the bay leaf and fresh thyme as you layer up the ingredients. Season well.
Arrange the remaining potato slices on top of the meat and brush with the melted butter. Cover with a tight-fitting lid or foil and cook in the oven for 1½ hours.
Remove the lid or foil and cook for a further 35 minutes, until the potatoes are golden and crisp. Serve with greens such as broccoli. Garnish with thyme.