Heat oven to 375°F. In large bowl, toss potatoes, 1 tablespoon of the oil, 1 teaspoon of the salt and 1/4 teaspoon of the pepper. Spread on cookie sheet with sides. Roast 20 to 25 minutes, stirring occasionally, until tender.
Meanwhile, sprinkle beef steak with remaining 1 teaspoon salt and 1/4 teaspoon pepper. In 12-inch nonstick skillet, heat remaining 1 tablespoon oil over medium heat. Add beef; cover and cook 8 to 10 minutes, turning once, until desired doneness.
Remove beef from skillet to cutting board; cover with foil and let rest 5 minutes.
Cut beef into thin bite-size pieces. In large bowl, toss beef, potatoes, spinach, tomato and dressing.