Cook the potatoes in a saucepan 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 ricotta. Season and set aside to cool completely.
Pat the prawns dry with paper towel, then chop roughly. Mix with the crab, chopped spring onions and Tabasco.
Put the spinach in a covered bowl with just the rinsing water clinging to the leaves. Cook in the microwave until just wilted. Cool, then squeeze dry and chop roughly. Mix the potato with the seafood and spinach. Chill for 30 minutes.
Preheat the oven to 190 degrees C. Shape the seafood mixture into four 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 a 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 soft. Allow to stand for 3–4 minutes before serving. Serve with lemon wedges.