Mix together the marinade ingredients in a bowl, add the lamb, cover and leave for 2 hours or overnight in the fridge.
To cook, place each fillet on a large square of cooking foil. Cover the fillet with a layer of tomatoes (both type) follow this with a layer of mozzarella then a second layer on top and pour over the marinade.
Wrap the lamb in the foil to create a sealed package, place on a baking tray and cook at 200C/400F/Gas 6 for 1 hour. Allow to rest in the foil for 10 minutes before slicing.
For the sauce: drain the cooking juices into a pan, add the stock and reduce to about one third. Thicken to preference with the cornflour. Serve.
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