Methods

1. Pat dry 1½ pounds skinless firm, flaky white fish fillets and cut crosswise into 4-inch pieces. Transfer the cut fish to a bowl along with 1 tablespoon lemon juice, 1 teaspoon fine sea salt, ½ teaspoon freshly ground white pepper, and ¼ teaspoon ground turmeric, and gently toss to coat the fish in the spices. Cover and refrigerate while preparing the sauce.
2. Heat 3 tablespoons extra virgin coconut oil in a Dutch oven or large, heavy-bottomed pot over medium heat. When the oil is shimmering, add 5 lightly crushed green cardamom pods, 4 whole cloves, 1 teaspoon black mustard seeds, and 12 curry leaves — be careful, the curry leaves crackle and pop! — and cook, stirring occasionally, until the spices smell fragrant and curry leaves turn bright green and translucent, 1 to 3 minutes. 
3. Add 3 large thinly sliced shallots, 4 finely chopped garlic cloves, 1-inch piece finely chopped ginger, and 1 to 2 quartered serrano peppers, depending on your desired heat level, and cook, stirring occasionally, until the shallots soft and start to turn translucent but don’t take on any color, 4 to 7 minutes. Add 12 to 15 cherry tomatoes, the remaining ¾ teaspoon salt, and ¼ cup water, stir, cover and cook, stirring occasionally, until the tomatoes just start to burst, 5 to 8 minutes. 
4. Add one 14-ounce can full-fat coconut milk, bring to a simmer, cover, and continue to cook until the tomatoes are easily crushed with the back of a spoon. Crush half the tomatoes, then nestle the fish into the sauce, spooning some of the sauce over each piece. Reduce the heat to medium-low, cover and cook until the fish is cooked through and easily flakes when pierced with a fork, 5 to 9 minutes. Taste and season with salt, if necessary.
5. Turn off the heat, gently stir 1 teaspoon apple cider vinegar, and tear the remaining 12 curry leaves, sprinkling them over top. Serve immediately with steamed short-grain rice or Appam.