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.