Heat the oil in a large sauté pan over medium heat. Add the onion slices and cook on both sides until they are a very deep brown, 15-20 minutes.
Coarsely chop the onion. Combine the tomatoes, onion, chile, and 1/4 teaspoon salt in a blender or food processor. Purée the mixture in short pulses to make a relatively even purée. Taste and season with salt and chipotle (and 1/4 teaspoon adobo sauce, or to taste, if using).
Serve warm or at room temperature with tortillas or chips.
Yield: 3 cups, 6 servings
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Sauces, Spreads, and Spices