Pat the steaks dry with paper towels and season well salt and freshly ground black pepper. Heat the oil in a heavy-bottomed cast iron skillet over medium-heat until just smoking.
Add the steaks and brown well, undisturbed, about 3 to 4 minutes. Flip and repeat until browned, about 3 minutes more for medium-rare. Remove to a cutting board, tent with foil, and let rest for 5 minutes.
Meanwhile, return skillet to stove, add the shallots and cook until softened, about 1 minute. Stir in the beer and cook until reduced slightly, about 1 minute. Stir in any accumulated meat juices and mustard. Remove the pan from the heat and add the butter. Swirl the pan to melt the butter or use a whisk to incorporate. Season with salt and pepper, to taste, and arrange the steaks on serving plates. Pour the sauce over the steaks and serve immediately.
Recipe courtesy of Aida Mollenkamp