In a Dutch oven or large pot, heat the oil over medium-high heat. Add the onions and cook, stirring until translucent and slightly caramelized, about 5 minutes. Add the garlic and ginger and cook, stirring for 1 minute. Add the curry powder and cook, stirring constantly, until the curry is lightly toasted, about 2 minutes. Add the lemongrass sachet, fish stock, and coconut milk and bring to a boil. Cook until the volume is reduced by half, about 20 minutes. Reduce the heat to medium, season the snapper on both sides with 3 teaspoons of the salt, and add to the pot with the brown sugar, red and yellow bell peppers. Cook for 10 minutes. Add the cilantro, stir, and remove from the heat. Cover to keep warm until ready to serve.

Meanwhile, in a 2-quart saucepan, combine the water, rice and remaining 1/2 teaspoon of salt, and bring to a boil. Cover and reduce the heat to low. Simmer until the liquid is absorbed and the rice is tender, 15 to 20 minutes. Remove from the heat and let sit for at least 5 minutes before fluffing with a fork.

To serve, spoon the rice into large bowls and ladle the curry on top. Garnish with additional chopped cilantro and serve.