Arrange 3 bananas (unpeeled) and 1 pint blueberries on a parchment paper-lined baking sheet. Roast in a 350 degrees F oven until the berries burst and the banana peels are deep brown, 20 to 25 minutes. Remove from the oven and let cool slightly, then peel the bananas. Puree the fruit in a food processor until smooth. Let cool, then refrigerate in an airtight container for up to 3 days or freeze for up to 1 month.
Photograph by John Lee by Arrangement with Rodale