Preheat the oven to 200°C, fan 180°C, gas mark 6. Cut a piece of kitchen foil to twice the length of a large roasting tin. Lay the foil inside the tin and lay a piece of baking parchment on top of it.
Place the potatoes in the lined tin with the lamb, onions, cumin, oregano, cinnamon, half the parsley and all of the stock. Mix well.
Scatter the tomatoes over the top, season with freshly ground black pepper, then fold over the foil and baking parchment and secure by crimping the edges. Bake for 1 hour.
Cut off and remove the top of the foil and baking parchment (to open up the parcel) then return to the oven and bake for a further 30 minutes or until the lamb is tender and the vegetables are cooked.
Remove from the oven, sprinkle over the remaining parsley and serve.
Cook's tip: For added Mediterranean flavour, top the finished kleftiko with slices of grilled halloumi cheese.