Combine the tomatillos, onion, serrano (including the seeds), garlic, cilantro stems and 1 teaspoon salt in a medium saucepan. Add enough water to just cover the vegetables and bring to a boil over medium-high heat. Reduce the heat and simmer until the tomatillos are tender and slightly darkened, about 15 minutes. Let cool slightly.
Transfer the vegetables to a blender with a slotted spoon; reserve the cooking water. Add the cilantro leaves and puree until smooth, adding a few tablespoons of the reserved cooking water if the salsa is too thick. Let sit 30 minutes to let the flavors develop. Add lime juice and salt to taste.
Photograph by Levi Brown