Preheat the broiler. Place the tomatoes and tomatillos on a baking sheet. Broil, turning occasionally, until charred all over, 10 to 12 minutes. Transfer to a saucepan along with the remaining ingredients. Bring the mixture to boil and cook until the onions are soft, 12 to 15 minutes. Transfer to a food processor or blender. Puree and then strain. Serve at room temperature or slightly chilled. Arbol salsa can be stored in the refrigerator 3 to 5 days or frozen for weeks. Just before serving, stir in the avocado.
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