Lightly oil a large rimmed baking sheet. Spread the tomatoes, onion, chili pepper, and garlic on the baking sheet. Drizzle with olive oil, and sprinkle with seasonings. Stir to coat evenly.
Roast until tomatoes and onions soften, stirring occasionally, about 20 minutes. Transfer to a blender and add cilantro. Blend until smooth.
Notes
This recipe makes enough for the chilaquiles as well as extra for serving.