Heat three tbsp. of oil in a heavy bottom skillet. Add garlic and sage sprig and cook over low heat until the sage starts to sizzle. Remove the garlic and discard. Drain the liquid from the cannellini beans and add cannellini beans to the skillet. Cook for one minute and add water. Season with salt and pepper.
Cook until all the liquid has been absorbed. Remove and discard the sage. Pour the beans into a bowl and set aside to cool.
Flake the tuna, removing any excess oil from the can. Gently mix in the tuna while the beans are still warm. Sprinkle with fresh sage, drizzle with olive oil and serve.