1–2 large chipotle chiles in adobo, roughly chopped
2tablespoons crushed piloncillo sugar (about 1½ounces), or firmly packed dark brown sugar
2Roma tomatoes, cut into eighths
Heat the olive oil in a medium sauté pan over medium heat. Add the onion and cook, stirring often, until it is soft but not brown. Add the garlic and cook 1 minute longer.
Add 1 of the chipotles, the piloncillo, and the tomatoes. Cook, stirring often, until the tomatoes are soft and the sugar almost melted, about 3 minutes. Add the water, tomato sauce, 1tablespoon of the vinegar, and ½teaspoon salt, and simmer over low heat for 5 minutes. Transfer the sauce to a blender and pulse several times to puree—leave a little bit of texture.
Taste the sauce. If it is too sweet for your taste, add the remaining 1teaspoon vinegar and another pinch of salt. If you want more heat, add the second chipotle and puree briefly.