To soak the beans, place the beans into a large bowl and cover with water. Soak overnight. Peel the onion and slice it on really thin slices. Dice the smoked pancetta. In a pot, warm the olive oil over moderate heat, add the onion and pancetta to the pot. Saute until lightly golden, around 5 minutes.
Peel the potato and slice it on thin slices. Add to the onion and pancetta, slice the garlic and add it to the pot. Also add drained beans and white wine, stir well and pour in 1 liter of water. Cook for around 30 minutes over moderate heat.
Add sage and bay leaf. Season with salt and pepper and cook for another 30 minutes or until the beans are tender. Strain the soup, put the liquid aside and blend the rest. Additionally start adding back the liquid until you get a nice, creamy texture. Add the milk and blend again. Season to taste.
Serve this creamy bean soup with crusty homemade croutons, a dollop of sour cream / creme fraiche and a bit of pan fried pancetta.