Fry onion in a little oil until soft. Add lamb, thyme, rosemary, garlic, paprika and wine.
Cook until the wine has mostly evaporated, then add the lamb stock, tomatoes and gently simmer until the sauce has reduced.
Season to taste and mix cornflour with a little water. Add to the mix to thicken.
Place lamb mix in an oven proof dish. Cover with the layer of mashed potatoes, sprinkle over the grated cheese and cook in the preheated oven until cheese is golden, about 30 minutes.