In a large, deep skillet, heat the oil. Add the garlic, shallots, chiles, lemongrass and ginger and cook over moderate heat, stirring occasionally, until the aromatics are softened and golden, about 5 minutes. Add the brown sugar and coriander and cook until the sugar is melted, about 20 seconds. Add the coconut milk and simmer, stirring, for 2 minutes. Transfer to a food processor and blend until pureed. Blend in the lime juice and fish sauce. Scrape the sauce into a glass measuring cup and season with salt and pepper. Leave the skillet on the stove.