Soak the beans overnight in a large bowl of cold water.
The next morning, preheat the oven to 400 degrees.
Drain the beans, rinse them very will and place them in a heavy oven proof casserole.
Pour in the water, then add the sage along with the olive oil, garlic, pancetta or prosciutto and fresh black pepper to taste.
Cover the casserole and bake for about 2 hours, stirring the beans twice. (The cooking time for beans can vary a lot, depending on their dryness and the method that was used to dry them).
When the beans are almost cooked, place the tuna in a small crockery or glass bowl, squeeze the juice of the lemon over it and mix very well.
Remove the cooked beans from the oven, add tuna mixture and salt and pepper to taste, and bake uncovered, for 10 minutes.
Remove from the oven and serve with freshly ground black pepper and parsley sprinkled over each serving.