1Preheat the oven to 350ºF. In the bowl of a food processor, combine the flour, oats, baking powder and salt; pulse until the oats are finely ground. Transfer the mixture to a mixing bowl.
2Put the beans, sugar, oil, egg and vanilla into the food processor and pulse until very smooth; add to the dry ingredients and stir just until the dough starts to come together.
3With dampened hands, shape the dough into a log on a parchment-lined baking sheet, flattening it so that it’s about 15 inches long and 3-4 inches wide. (Divide the dough into two if it doesn’t fit on your cookie sheet, or make longer, thinner logs for smaller biscotti.) If you like, sprinkle the top with coarse sugar.
4Bake for 25-30 minutes, until golden and firm. Transfer to a wire rack to cool, then cut on a slight diagonal into 1/2-inch thick slices using a serrated knife. Stand the biscotti upright on the baking sheet, spacing them about half an inch apart, and bake for another 30 minutes, until crisp and dry. Makes 1 1/2-2 dozen biscotti.