In a blender or food processor, combine the drained white beans.
Add about half of the reserved liquid and blend until completely smooth.
Add more liquid, one tablespoon at a time, until you reach a smooth, thick consistency that isn't watery.
Transfer the purée to a medium saucepan and add the butter or olive oil.
Place over medium-low heat and stir gently until the butter is melted and the purée is heated through.
Season generously with salt and freshly ground black pepper to taste.
Garnish with chopped fresh parsley and serve hot as a side dish, or warm/room temperature as a dip or spread.
Roasted Garlic & Herb: Add 15-20 cloves of roasted garlic and 1 teaspoon (5 ml / 0.2 fl oz) of fresh thyme or rosemary to the blender with the beans for a deep, aromatic flavor.