This recipe is from Hugo Ortega’s Street Food of Mexico cookbook. Find several more recipes from the book here.

4 medium tomatillos, husks removed,
  coarsely chopped

1 serrano pepper, stemmed
2 garlic cloves, peeled
1 tablespoon finely chopped white onion
¼ small bunch cilantro
¼ teaspoon kosher salt

Place tomatillos, pepper, garlic,and onion in a blender and purée until smooth. Add cilantro and salt; blend for only a couple of seconds, to keep some cilantro leaves whole.