Preheat the oven to 400 degrees F. Place the green beans and onion on 1 baking sheet and the carrots and potatoes on another; toss each with 1/2 tablespoon olive oil and 1/2 tablespoon mustard. Season with salt and pepper. Roast until tender and crisp, about 12 minutes for the beans and onion, and about 20 minutes for the carrots and potatoes.
Sprinkle the steak with the Italian seasoning and 1/4 teaspoon each salt and pepper. Heat the remaining 1 tablespoon olive oil in a large skillet over medium-high heat. Add the steak; cook 3 to 4 minutes per side for medium rare. Transfer to a cutting board and let rest 5 minutes.
Let the skillet cool slightly, then add the vinegar, the remaining 1/2 tablespoon mustard and 1/4 teaspoon salt. Cook over medium heat, scraping up any browned bits, about 1 minute.
Toss the romaine and endive with the roasted vegetables and all but 1 tablespoon of the vinegar mixture in a large bowl. Slice the steak; serve over the salad. Drizzle with the reserved vinegar mixture.