Heat oven to 425° F. Line a rimmed baking sheet with parchment. Whisk together the mustard, lemon juice, honey, and 2 tablespoons of the oil in a small bowl; set aside.
Toss the potatoes, thyme, 3 tablespoons of the remaining oil, and ¼ teaspoon each salt and pepper in a medium bowl. Arrange the potatoes in 4 piles of overlapping slices on the prepared baking sheet. Roast until brown and tender, 25 to 30 minutes.
Heat the remaining tablespoon of oil in a large skillet over medium-high heat. Season the steak with ½ teaspoon salt and ¼ teaspoon pepper. Cook until an instant-read thermometer inserted in the thickest part registers 130° F, 3 to 4 minutes per side for medium-rare.
Serve the steak and potatoes alongside the romaine, drizzled with the dressing.