Put the beans, potatoes and sausage in a tall saucepan. Fill up with water so that everything is well covered. Add the vegetable stock, savory and butter and bring to the boil in a closed saucepan. Let the soup simmer for about 1 hour. Mash the potatoes and beans with a pounder and then thicken the soup with a little roux. Finally season with salt and pepper. Serve with a dash of vinegar, depending on your taste.