In a large pot set over a high heat, add the cardamom pods, ginger, garlic, cinnamon stick and 1 litre/35 fl oz/4¼ cups water and bring up to the boil. Reduce the heat to low and add the salmon, ensuring it is completely covered with the liquid.
Cover with a lid and simmer gently for 5 minutes or until the salmon is cooked through and tender. Remove the salmon from the liquid and set aside to cool.
Put the vegetable oil in a large non-stick frying pan and set over a medium-high heat. When hot, add the green chilli and yellow mustard seeds and fry until the seeds pop and turn dark brown.
Turn the heat down to medium and add the chopped tomatoes, ground turmeric, chilli powder and black treacle and continue to cook for 5 minutes.
Add the spinach, lemon juice and salt. Once the spinach has wilted down (about 3 minutes), add the poached salmon and cook for 5 minutes, or until the salmon is cooked through.
Finish with the pomegranate seeds and fresh coriander just before serving.