Heat the oil in a large pan, then fry the cinnamon, the cloves and the seeds. When the mustard seed start popping, add the curry leaves and the onion, and gently cook, stirring constantly.
When the onion is soft, add the garlic and ginger, plus a spoonful of water, then stir for one minute.
Season with salt, add the turmeric and pour in 100 ml water, then cook for 7 to 8 minutes.
Add the coconut milk, bring to a simmer and cook for 3 minutes.
Taste it - if necessary, season with salt and pepper. If the sauce is too thick, add some water (it should be creamy).
Add the fish pieces, placed in the pan side by side. Cook under cover for 5 minutes. Turn off the heat and stir in the lemon juice.
Serve hot.