Use to make Chicken Tomatillo Tostadas.
In a medium saucepan, heat olive oil over medium-high heat. Add onions and bay leaf; season with salt and pepper. Cook, stirring, until onion is translucent, about 6 minutes. Add garlic and chili powder; cook, stirring, until garlic is softened and mixture is fragrant, about 3 minutes.
Remove bay leaf and discard. Add 2/3 of the black beans and stir to coat. Add 2/3 cup water and mash mixture using a potato masher. Add remaining black beans and stir to combine; add water, if necessary to thin. Season with salt and pepper and keep warm until ready to use.