Season the steak with ¾ teaspoon kosher salt and ¼ teaspoon pepper. Cook in the olive oil in a large skillet over medium-high heat, 3 to 5 minutes per side for medium-rare. Let rest 5 minutes, then slice against the grain.
In a small bowl, whisk together the mustard, vinegar, and shallot.
Meanwhile, steam the green beans until tender, 4 to 6 minutes.
Top the steak with the mustard sauce and serve with the green beans.