1 kg fresh tomatillos, husked, rinsed, and quartered
1 fresh serrano chile, seeded and chopped
1/2 large white onion, cut into 4 wedges
2 garlic cloves, chopped
1/2 cup water
1/2 cup chopped coriander
1 tablespoon fresh lime juice
Coarsely purée tomatillos, chile, onion, garlic, water, and 1 teaspoon salt in a blender. Transfer to a large heavy skillet and simmer, stirring occasionally, until slightly thickened, about 15 minutes. Transfer to a bowl and cool to room temperature, then stir in cilantro, lime juice, and salt to taste.