Preheat oven to 450°.
Heat a large nonstick skillet coated with cooking spray over medium-high heat. Add onion and mushrooms; cover and cook 3 minutes. Uncover and cook 5 minutes, stirring occasionally. Stir in 1/4 teaspoon salt; remove onion mixture from pan.
Sprinkle steak with 1/2 teaspoon salt and pepper. Add steak to pan; cook over medium-high heat 3 minutes on each side or until desired degree of doneness. Let stand 5 minutes; cut steak diagonally across grain into thin slices.
Place crusts on a baking sheet. Bake at 450° for 3 minutes. Remove crusts from oven.
Combine mayonnaise and horseradish; spread each crust with about 2 teaspoons mayonnaise mixture. Arrange onion mixture evenly over crusts; bake at 450° for 2 minutes. Divide steak and cheese evenly among pizzas; bake an additional 2 minutes or until cheese melts. Cut each pizza in half.