Toast one slice of Italian bread.
Meanwhile, season and cook steak to your desired doneness.
Heat a small amount of butter in a nonstick skillet over medium-high heat until hot.
Break egg gently into pan and immediately reduce heat to low. Cook slowly until whites are completely set and yolks begin to thicken but is not hard, about 5 to 6 minutes.
Spread horseradish onto toasted Italian bread. Top with sliced plum tomatoes, steak, and sunny-side up egg.