Pour water into a saucepan, add garlic and bring to a boil over high heat. Add beans and return to a boil. Drain the beans and garlic, reserving ¼ cup of the cooking liquid.
Transfer the beans and garlic to a food processor. Add oil and process, adding just enough of the reserved cooking liquid to make a thick, smooth paste. Stir in sage and season to taste with salt and pepper.
Make Ahead Tip: Cover and refrigerate for up to 2 days.