Slice the halibut into large chunks, about 2 inches across. Sprinkle with salt and set aside. In a wok or large saute pan, heat the vegetable oil over medium-high heat for a minute and then add the onion, lemongrass, ginger and green chiles. Turn the heat up to high and stir fry for 2-3 minutes, until the onions begin to color.
Add the coconut milk and the curry paste, then turn the heat down to medium and stir well to combine. Add the fish sauce and the lime leaves, mix well and simmer — do not let it boil hard — for 5 minutes.
Add the halibut chunks and simmer for another 5-10 minutes. Serve with steamed white rice and top with the basil as a garnish. Try serving with a fruity white wine, such as an off-dry Riesling, a Gewurztraminer or a California Roussane. Enjoy!