In a small bowl, whisk together mustards, vinegar, and oil; season with salt and pepper. Set aside.
Heat broiler, with rack set 4 inches from heat. Line a rimmed baking sheet with foil. Place steaks on sheet; season with salt and pepper. Broil 4 to 6 minutes for medium-rare. Transfer to a cutting board; tent with foil. Let rest 5 minutes; thinly slice crosswise.
Meanwhile, in a medium saucepan of boiling salted water, cook green beans until crisp-tender, 4 to 6 minutes; drain. Run beans under cold water; drain well.
In a large bowl, combine dressing, steak, beans, lettuce, and tomatoes; toss. Sprinkle with cheese.