Soak the beans overnight in enough cold water to amply cover them mixed with the baking soda.
The next day, drain and place the beans in a measuring jug to get an estimate of their volume and transfer to a fireproof earthenware or enameled cast iron casserole dish with twice their volume of water.
Add the unpeeled garlic cloves, sage leaves but no salt, and bring slowly to a gentle boil.
Simmer the beans over very low heat for 2 1/2 hours or until they are tender, removing any scum from the surface at intervals.
After just over 1 hour's cooking time, stir in 2 1/2 tsp salt and 1 1/2 tbsp olive oil.
When they are cooked, remove and discard the sage leaves; squeeze the soft puree from the garlic into the beans and stir.
Add a little salt if necessary.
Trim and thoroughly wash the escarole; blanch for 2 minutes in a large saucepan of boiling salted water.
Drain well and place an equal amount in deep, heated soup bowls; ladle plenty of beans on top and sprinkle a little olive oil and freshly ground white or black pepper over them.