Place cut shallots, onion, garlic, ginger, and turmeric in a food processor, add 2 tbsps of oil to grease the blades, and blend until you get a fairly smooth paste. Add curry powder and chilli paste (add between 3 to 5 tbsps, depending on how spicy you like it) and pulse until well combined.
Heat up 6 tbsps oil in a wok over low-medium fire. When hot, add the spice paste and stir fry, stirring continuously around in the wok to avoid burning the paste. If the paste gets a little dry, drizzle in 2 - 3 tbsps of oil. Stir in salt and sugar. Allow the spice paste to cook through gently, becoming fragrant and aromatic - this may take 10 minutes or longer.
Once the oil starts to float to the top, and bubbles around the edges of the paste, stir in lemon grass and curry leaves. Stir fry for a minute or until fragrant.
Stir in thin coconut milk and tamarind juice. Increase heat to medium-high, and allow mixture to come to a gentle simmer. Add egg plant, and simmer for a few minutes. When egg plant start to soften, stir in the lady fingers, and tomatoes. Then add the fish head halves, cheek side facing up. Add a little extra water, if sauce becomes too thick, or is not enough to coat the fish head to cook it.
Cook for 12 to 15 minutes, or until fish head is cooked through. Do a taste test: if your prefer your curry more rich-tasting or creamy, add thick coconut milk to desired level of creamy richness. Add salt and sugar to taste (more salt if not salty enough, more sugar if too sourish or too salty). A good fish head curry should taste rich or lemak, slightly salty as well as sourish (tangy), and have the consistency of a thick soup.