Measure out 1 tsp each cumin seeds and black onion seeds (keep the rest for later) and tip them into a large pan. Place on a high heat and toast, shaking the pan, for 1-2 mins till the spices smell aromatic and start to pop. Tip them into a bowl and set aside. Take the pan off the heat.
Peel and finely chop the onion. Peel the potato and dice it to match. Put the pan back on a medium-low heat, add ½ tbsp oil and the diced veg. Season with a pinch of salt and pepper and pop the lid on the pan. Sweat the veg, stirring every few mins, for 8 mins till they start to soften. If they start to stick, turn the heat down and add 2-3 tbsp water.
While the veg sweat, quarter the cabbage and slice out the thick cores. Finely slice the cabbage leaves. Fill your kettle and boil it.
Tip the toasted seeds into the veg and add ½ tsp turmeric. Fry, stirring, for 1 min. Add the cabbage to the pan. Pour in the coconut milk and 700ml boiling water. Turn the heat up and bring to the boil, then turn the heat down a little and simmer for 15 mins, without the lid on (this helps to keep the cabbage green).
When the cabbage is tender, take the soup off the heat and use a stick blender to blitz it smooth – or ladle the soup into a blender and blitz. Trickle in a little extra water to get a thinner consistency, if you think it needs it. Taste and add more salt or pepper if you think it needs it. Set aside for few mins while you fry the spices to top the soup.
Put a small frying pan on a medium-high heat. Add ½ tbsp oil and warm it for 1-2 mins. Add another 1 tsp each cumin seeds and black onion seeds to the pan, along with the curry leaves. Sizzle for 30 secs-1 min.
Ladle the soup into 2 warm bowls. Top with the fried spices and their oil. Serve straight away.