Heat the butter and the oil in a saucepan to medium heat. Add the onion and the garlic and gently fry them for 5 minutes.
Add the beans making sure to add some of the bean juice. Season them with salt and bring them to a boil.
Once they are boiling, mash them with the potato masher or blend them with a hand blender. Remember to leave some texture.
Serve them with crumbled queso fresco or feta, sour cream (optional). Perfect dish to have with any other wonderful Mexican dish.