Drain the beans and put in a flameproof casserole with a tight-fitting lid. Cover with stock or water to a depth of 5 cm (2 inches) above the beans and push in the sage leaves. Bring to the boil, then cover and bake in a preheated oven, 160°C (325°F), Gas Mark 3, for about 1 hour, or until tender. The beans may not take this long, depending on their freshness, so test after 40 minutes. Leave in their cooking liquid.
Put half the beans, the cooked sage leaves and all the cooking liquid into a food processor or blender and process until smooth. Pour the purée back into the casserole with the remaining beans. Add extra stock or water if too thick.
Heat the oil in a frying pan over a low heat. Add the garlic and cook, stirring, until soft and golden. Add the chopped sage or rosemary and cook, stirring, for 30 seconds. Stir into the soup and reheat until boiling, then simmer gently for 10 minutes. Season well with salt and pepper. Serve immediately, garnished with chopped parsley and drizzled with Toasted Garlic & Chilli Oil (see below), if liked.