Place chopped chiles, tomatillo, cilantro, garlic, and water in a small saucepan. Bring to a boil over high heat, reduce to a simmer, and cook until chiles and tomatillo are softened, about 10 minutes.
Transfer mixture to the jar of a blender and purée until completely smooth. Transfer to a bowl, stir in sour cream and lime juice. Season with salt to taste and serve immediately, or store in an airtight container in the refrigerator up to a week.
