Arrange spinach on a large plate. Sprinkle with cranberries and walnuts, and arrange tomato slices on top. Set aside.
In a non-stick skillet (or a regular skillet coated with non-stick spray) cook steak over medium heat until no pink remains and steak is thoroughly cooked.
Arrange cooked steak over salad. Sprinkle salt and pepper on top, and drizzle with your favorite dressing. Note: I suggest using a light flavored dressing. Citrus dressings taste especially good on this salad!