Method of Preparation
Preheat the broiler. Set aside half an onion, sliced into thin rings. Add the other onion half, husk tomatoes, and Serrano on a foil-lined baking sheet. Boil for 7-10 minutes, until the vegetables are soft and slightly brown. Transfer the vegetables to a blender, add the broth and purée. Season with ½ teaspoon salt and the sugar.