Freeze a batch of this tangy relish for a spur-of-the-moment starter over crostini or a main event with pasta.
In a 5-quart Dutch oven or pot, heat oil over medium-high. Add onions, raisins, pine nuts, garlic, and red-pepper flakes; cook, stirring occasionally, until onions are softened, 6 to 8 minutes.
Add tomato paste, cocoa powder, and 2 tablespoons sugar; cook, stirring, until tomato paste is fragrant, 2 to 3 minutes. Add eggplant, vinegar, and 2/3 cup water.
Cover, and cook over medium-high, stirring occasionally, until eggplant is tender and mixture is thick, 7 to 10 minutes. Season with salt and more sugar, as desired. Serve caponata warm or at room temperature.
To freeze, cool completely, then transfer to resealable freezer bags, filling only halfway; freeze bags flat, and store in the freezer, up to 2 months.