firm white fish, such as whiting, pollock, or monkfish • optional: 4 king prawns, skin on • neutral oil, like groundnut or sunflower • black mustard seeds • about 15 fresh curry leaves • onion or 2 shallots, finely chopped • thumb-size piece of fresh ginger, peeled • Optional: 2 knobbly little pieces fresh turmeric, peeled • garlic cloves, peeled • ground turmeric • chilli powder • tomato paste or 1 fresh tomato, finely chopped •