How to Make It
In a mini food processor, combine the chopped onion, chopped garlic and lemongrass, and process to a paste.
In a large nonstick skillet, heat the vegetable oil. Add the onion paste and cook over moderately low heat, stirring occasionally, until golden brown, about 8 minutes. Add the ground coriander and cook until fragrant, about 2 minutes. Add the sambal oelek and cook, stirring, for 2 minutes. Add the coconut milk and boil over high heat until thickened, about 2 minutes. Stir in the tamarind concentrate, sugar and peanut butter until smooth. Remove from the heat and season with salt. Let cool, then transfer to a bowl.
Sambal oelek, a spicy Indonesian chile sauce, and tamarind concentrate are both available at Asian markets and at specialty-food stores.