1 pound plum tomatoes (4 to 5), cored
1 large white onion, sliced 1/2 inch thick
5 cloves garlic, unpeeled
1 canned chipotle chile pepper in adobo
1 teaspoon apple cider vinegar
Preheat the broiler. Arrange the tomatoes, onion and garlic close together on a rimmed baking sheet. Broil, turning occasionally, until the tomatoes burst and all the vegetables are softened and charred, about 15 minutes. Let cool 5 minutes.
Peel the garlic, then transfer to a blender along with the remaining broiled vegetables. Add 1/2 chipotle chile, the vinegar, 1 1/2 teaspoons salt and 1/2 cup water; puree until smooth. For more heat, add the remaining 1/2 chipotle and puree. If the salsa is too thick, add a splash of water. Let sit 30 minutes to let the flavors develop. Season with salt.