Soak the beans overnight and cook for about 1 hour without salt, then leave to cool.
Fry the sage leaves in hot olive oil until crispy. Add the peeled and finely sliced garlic cloves and fry briefly in the oil. Remove the pan from the heat, add the stock and vinegar and season with salt, pepper and sugar. Pour the marinade over the beans, mix well and let it steep, possibly overnight.
Tastes wonderful as a starter with other antipasti such as roasted red peppers, ham with melon, roasted eggplant, celery, squid salad, etc.
Tip for those in a hurry:
The recipe also works with canned beans, of course, but home-cooked beans are better. 1 large can of white beans is a good starter for 4 people.