In a small bowl combine beets and horseradish to make spread; blend well. To assemble sandwich, spread beet-horseradish mixture on one side of 4 slices of bread. Arrange beef, onion, mushrooms and lettuce on top. Top with remaining slices of bread.
Nutritional Information Per Serving: Calories 260; Total fat 3g; Saturated fat 1g; Cholesterol 20mg; Sodium 900mg; Total carbohydrate 41g; Fiber 4g; Protein 18g; Vitamin A 40%DV*; Vitamin C 15%DV; Calcium 6%DV; Iron 20%DV; Folate 22%DV; Potassium 7%DV *Daily Value.