Mix the turmeric, yoghurt and salt together in a medium-sized bowl. If you’d like the cabbage as ‘steaks’, cut the cabbage into four thick wedges, including the core. For a ‘pulled cabbage’ curry, remove the core and separate the leaves. Add the cabbage leaves or wedges to the turmeric mixture and stir to coat well.
For the curry paste, heat the coconut oil in a wide heavy-based frying pan over medium–low heat. Add the garlic and ginger and gently fry, stirring, for 2–3 minutes, or until a light golden brown. Add the remaining curry paste ingredients and cook for another 1–2 minutes, until aromatic and darkened slightly. Transfer to a blender with ⅓ cup (80 ml) water and whiz until smooth.
Place the pan back over medium heat and add the coconut oil. When the oil is hot, add the curry leaf branches and cook for 1–2 minutes, or until crisp. Remove using tongs and drain on paper towel. Add the curry paste to the pan, along with the onion, green chilli and potato and cook, stirring frequently, for 4 minutes, until caramelised. Stir in the coconut milk, sugar and 1 cup (250 ml) water and bring to a gentle simmer. Cook for 10 minutes, or until the potato is half-cooked.
Add the cabbage, with the marinade. Cover and cook, stirring or turning the cabbage halfway through, for 15 minutes, or until the cabbage has softened and the potato is cooked through.
Stir in the lemon juice and peas, then remove from the heat. Cover and stand for 5 minutes to warm through.
To serve, scatter the curry with the crispy curry leaves, nigella seeds and extra chilli, and an extra drizzle of coconut milk. This curry yields quite a bit of gravy, so you may like to serve with basmati rice and flatbreads, or on its own as more of a soup.