Steak coated with a tangy mixture of whole-grain mustard and black pepper is a delicious contrast to the sweet roasted vegetables.
Preheat oven to 475 degrees. In an 11-by-15-by-2 3/4-inch roasting pan, combine mushrooms, squash, and onion. Add 1 tablespoon oil and 3/4 teaspoon salt; toss to coat. Arrange vegetables around sides of pan; place steak in center of pan.
In a small bowl, stir together mustard and peppercorns. Coat steak with the remaining 1 1/2 teaspoons oil, and sprinkle generously with salt. Spread mustard-pepper mixture over top of steak; press down to adhere.
Roast, turning and stirring vegetables once, until steak is lightly browned and an instant-read thermometer inserted in one side of steak registers 130 degrees to 135 degrees for medium-rare, 15 to 20 minutes.
Transfer steak to a cutting board to rest; tent loosely with aluminum foil. Spread vegetables in an even layer, return to oven, and continue roasting until lightly browned and tender, 8 to 10 minutes. Cut the steak into 1/4-inch thick slices, and serve with vegetables.