How to Make It
In a food processor, combine the fish sauce with the thinly sliced lemongrass and the shallots, garlic, sugar and curry powder; process to a fine paste.
In a medium saucepan, heat the oil. Add the onion and cook over moderately low heat until softened and starting to brown, about 7 minutes. Stir in the coconut milk, tomatoes, turmeric and the spice paste and bring to a boil. Add the seasoning sachet and simmer over low heat, stirring occasionally, until the sauce coats the back of a spoon, about 15 minutes. Let cool slightly, then pull out and discard the sachet. Transfer the contents of the saucepan to a blender. Add the peanuts and blend to a puree. Season with salt.