Cook the potatoes in a pan of boiling water for 12–15 minutes until tender. Drain and return to the pan over the heat to dry out a little, then mash until smooth. Beat in the soft cheese or ricotta. Season and set aside to cool.
Pat the prawns dry with kitchen paper, then chop roughly. Mix with the crab, chopped onions and spicy sauce.
Cook the spinach in the microwave in a covered bowl until just wilted. Cool, then squeeze dry and chop roughly. When the potato mixture is cold, mix with the seafood and spinach. Chill for about 30 minutes.
Preheat the oven to 190°C. Shape the seafood mixture into 4 large cakes, dipping your hands in a little flour if necessary.
Put the flour in a shallow bowl, the beaten egg in another shallow bowl and the breadcrumbs mixed with the dried herbs in a third. Coat the fishcakes evenly first in flour, then in beaten egg and, finally, in the crumbs.
Halve the tomatoes and place around the fishcakes on the baking tray. Season and sprinkle with any leftover crumbs. Brush the fishcakes with the warmed oil, then bake for 20 minutes until they are crisp and the tomatoes softened. Allow to stand for 3–4 minutes before serving.
Use flaked cooked cod, haddock or salmon instead of the crab and prawns.