1. To prepare cream, combine horseradish, sour cream, and crème fraîche in a small bowl; cover and chill.
2. Preheat broiler.
3. To prepare sandwich, heat a large nonstick skillet over medium heat. Coat pan with cooking spray. Add onion and 1/8 teaspoon salt to pan; cover and cook 10 minutes, stirring frequently. Uncover and cook 10 minutes or until onion is golden brown.
4. Sprinkle steak with 1/8 teaspoon salt and pepper. Place steak on a broiler pan coated with cooking spray; broil 5 minutes on each side or until desired degree of doneness. Let stand 5 minutes. Cut steak diagonally across the grain into thin slices. Combine steak and remaining 1/4 teaspoon salt in a bowl, and toss to coat.
5. Combine arugula, oil, and vinegar in a medium bowl; toss gently. Spread 2 tablespoons horseradish cream over each bread slice; top each with about 1/3 cup onion, 1/2 cup arugula mixture, and 3 ounces of steak.