Heat the oil in a large heavy sauté pan over high heat. Add as many shanks as will comfortably fit and brown on all sides. Transfer to a deep wide baking dish or roasting pan with a lid or a Dutch oven. Repeat with the remaining shanks.
Heat half the butter in another sauté pan over medium heat. Add the onions and cook until translucent, about 10 minutes; transfer to the dish with the s