1. First make the sauce. Whiz or liquidise the tomatoes and garlic, strain into a small bowl and stir in the yoghurt and mint, season to taste with a pinch of salt, pepper and sugar. Set aside while you make the fish cakes.
2. Place a wide frying or sauté pan on the heat, allow to get slightly hot, then add half the oil, fish and ground coriander. Toss in the pan for 2 minutes until the fish is just opaque. In a bowl, mix the remaining ingredients, except the egg, and add the cooked fish. Add enough beaten egg to bring it together to a softish mixture (if the egg is very large you may not need it all). Season to taste, then shape it into 4 fish cakes in your hands.
3. For the coriander dressing, mix together the olive oil, lemon juice and coriander and season to taste, adding more lemon juice if necessary.
4. To cook the fish cakes, place a frying pan on a medium heat, heat the remaining oil and put in the fish cakes. Turn down the heat slightly and cook gently for 3-4 minutes on each side until golden brown on both sides. Place the fish cakes on two warmed plates and serve with the tomato, yoghurt and mint sauce and the watercress salad drizzled with just enough dressing to make the leaves glisten.