SALSA RANCHERA

1 medium white onion, quartered
3 lb ripe medium tomatoes, cored
1/2 cup olive oil
1 fresh serrano chile, chopped, including seeds
1/2 cup chopped fresh cilantro
3 tablespoons red-wine vinegar 
Accompaniment: corn tortilla chips 

Preheat oven to 400°F. Arrange 2 onion quarters and 2 pounds tomatoes in a small roasting pan and drizzle with 1/4 cup oil. Arrange remaining 1 pound tomatoes in a small baking pan and drizzle with remaining 1/4 cup oil. Bake both pans of tomatoes in upper and lower thirds of oven, switching position of pans after 45 minutes, until browned, about 1 1/2 hours. Remove roasting pan and continue baking small pan of tomatoes until blackened, about 30 minutes more. Transfer all roasted tomatoes (do not peel) and onions with a slotted spoon to a food processor, discarding liquid in pans. If your processor is small, do this in 2 batches. Pulse until coarsely chopped. (Alternatively, you can chop with an immersion blender in a deep container.) Chop remaining 2 onion quarters and stir together with tomato mixture, serrano, cilantro, and vinegar. Season with salt and chill, covered, at least 8 hours to blend flavors. Bring to room temperature before serving.