Heat oil in a medium
Dutch oven or other heavy pot over medium heat. Add onions and cook, stirring, until just beginning to turn golden brown, 10–14 minutes. Stir in beans and add
bay leaf. Cook, stirring occasionally, until beans begin to soften and fall apart, 9–11 minutes. Season beans with salt and remove from heat. Remove and discard
bay leaf. Mash beans with a
potato masher until smooth and spreadable.