In a medium bowl, combine the fish chunks with 1/4 teaspoon turmeric, lemon juice, and salt. Toss gently to coat, then let it rest for 15 minutes. You will see the fish turn a beautiful bright yellow; this builds a savory foundation and firms up the flesh.
Heat 1 tablespoon of coconut oil in a wide pan over medium heat. Sear the marinated fish pieces for 1-2 minutes on each side. You want to see the edges turn opaque and form a very light, pale crust. Carefully remove the fish and set it aside; this prevents the fish from disintegrating later in the gravy.
Add the remaining 1 tablespoon of coconut oil to the same pan. Stir in the mustard seeds. Once you hear them pop and crackle, scatter in the curry leaves, slit green chilies, ginger, and garlic. Sauté for 1 minute until highly aromatic. Add the sliced onions and cook until they are soft and translucent, making sure they do not turn brown.
Stir in the remaining 1/4 teaspoon of turmeric and pour in the 1/2 cup of thin coconut milk. Bring it to a gentle, bubbling simmer. This liquid acts as the cooking medium to extract all the sweet onion and spicy ginger juices.
Carefully slide the seared fish pieces and the tomato wedges into the simmering pan. Cook uncovered on medium-low heat for 5-6 minutes. You will see the tomatoes soften slightly and the fish turn perfectly tender and flake easily.
Lower the heat to low. Pour in 1 cup of thick coconut milk. Swirl the pan gently to blend rather than using a spoon. Let it heat through for 2 minutes without boiling. Turn off the heat, stir in the fresh lime juice, and watch the gravy turn incredibly glossy and velvety.