1/2 pound fresh tomatillos, husked, rinsed, and quartered1 1/2 pounds tomatoes, chopped, divided 1/2 cup chopped white onion, divided 1 fresh serrano chile, coarsely chopped, including seeds 1 garlic clove, quartered 2 tablespoons red-wine vinegar 2 tablespoons olive oil 1/2 cup chopped cilantro 1 1/4 tsp salt

Puree tomatillos, half of tomatoes, and half of onion with chile, garlic, vinegar, and 1 1/4 teaspoons salt in a blender until smooth.  Force through a medium-mesh sieve into a bowl, discarding solids.  Stir in remaining tomatoes and onion, oil, and cilantro. Chill until cold, at least 1 hour and up to 4 hours.  Add a squeeze of lime juice just before serving.

adapted recipe from epicurious.com