Preheat the oven to 220ºC, fan 200ºC, gas 7. Heat a large non-stick frying pan and cook the lamb on a high heat in batches, ensuring you get a good golden crust on the mince.
Transfer each batch to a bowl, then clean the pan. Sweat the onions for 10 minutes, covered. Add the cinnamon and garlic, and cook for a minute. Add the tomatoes and stock to the onions, then transfer the mixture to a saucepan. Cover and cook for 40 minutes. Leave to cool.
Meanwhile, place the aubergines and peppers on 2 baking trays, brush with oil and season. Cook in the oven for 45 minutes, swapping the trays round halfway through.
Remove the aubergines and peppers from the oven and reduce the temperature to 200ºC, fan 180ºC, gas 6. Assemble the moussakas in 2 x 2 litre ovenproof dishes. Divide half the lamb mixture and all the tomato mixture between the 2, layer over the aubergines and peppers and spoon over the remaining lamb. Mix together the watercress sauce and egg yolks and drizzle over the top. Crumble over the cheese and bake one for 40 minutes.
To freeze the other dish, cool it completely and freeze for up to 1 month. Defrost and cook as before, or cook from frozen, doubling the cooking time. Check that it's piping hot in the middle before serving.