How to Make It
Preheat oven to 400°F. Season steak with 1/2 teaspoon each salt and pepper; set aside. Place sweet potatoes on a rimmed baking sheet and roast for 20 minutes.
Toss Brussels sprouts with 2 tablespoons oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a bowl. Add to baking sheet with sweet potatoes and continue to roast, shaking sheet halfway through, until potatoes are tender and Brussels sprouts are golden brown, 20 to 25 minutes more.
Meanwhile, heat remaining 2 tablespoons oil in a large skillet over high. Cook steak, turning once, until deeply browned, 5 minutes per side for medium. Transfer to a cutting board and let rest for 5 minutes before slicing.
Whisk mustard, vinegar, and honey in a large bowl. Add parsley and Brussels sprouts and toss to coat. Halve potatoes lengthwise and top each half with butter. Season with remaining 1/2 teaspoon salt and 1/4 teaspoon pepper.
Serve steak with Brussels sprouts and sweet potatoes.