Soak overnight in 4 cups of water
Drain and cover with fresh water by 2 inches in a saucepan. Bring to a boil, lower heat and skim off any foam. Simmer gently until tender - 1 hour or so (or more depending on age and size of beans)
Season with salt to taste. Set the beans aside to cool in their liquid
Finely dice onion, carrot and celery (or celeriac)
Heat the oil in a heavy-bottomed the pan.
Add the diced vegetables and cook until tender, about 10 minutes.
4 garlic cloves, thinly slices, 6 fresh sage leaves, chopped. Add salt to taste
Cook for 5 minutes and then stir in:
1/2 cup chopped tomatoes, fresh or canned.
Cook for 5 minutes. Taste and add salt if necessary.
Drain the beans, reserving the liquid. Mix the beans with the vegetables and put into a medium-size gratin or baking dish. Taste for salt. Add enough bean liquid to almost cover. Drizzle with olive oil and cover with toasted breadcrumbs
Bake for 40 minutes in a preheated 350F degree oven, checking occasionally. If the gratin is drying out, carefully spoon in a little bean liquid (pouring at the sides of the gratin dish to avoid getting the crumbs wet).