We like the economy, flavor, and tenderness of chuck steak, but sirloin or any other boneless steak, pounded thin, is excellent, too.
Toss together beans, shallot, 2 tablespoons vinegar, mint, and 2 tablespoons oil; season with salt and pepper. Let stand, tossing occasionally, at least 30 minutes and up to 2 hours.
Meanwhile, briefly pound steaks on both sides with the tenderizing side of a meat mallet; season with salt and pepper. Stir together hoisin and remaining tablespoon vinegar. Brush steaks with hoisin mixture; let stand at least 20 minutes and up to 1 hour. Heat remaining tablespoon oil in a large skillet over medium-high heat until shimmering. Add steaks and cook, flipping once, until edges are caramelized and deep brown, about 5 minutes for medium-rare. Let rest 10 minutes. Divide steaks and beans among 4 plates and garnish with mint leaves.