Cut cilantro stems into 2-inch lengths. Combine cilantro stems, garlic, ginger, and chiles in a blender, and process on medium speed until finely chopped, about 30 seconds, stopping to scrape down sides as needed. Add lime juice, fish sauce, and sugar to blender, and process on high speed until well combined, about 30 seconds. Stir in cilantro leaves and scallion slices.