Heat butter and oil in large skillet over high heat. Season beef with 1/4 teaspoon salt and 1/8 teaspoon pepper. Add to skillet; cook over high heat 3 minutes per side for medium-rare (140°F). Remove from skillet; keep warm. Do not drain skillet.
Add bacon to skillet; cook 3 minutes. Add 1/4 cup scallions and potatoes; cook 3 minutes, until browned. Season with 1/4 teaspoon salt and 1/8 teaspoon pepper.
Mix sour cream, yogurt, horseradish, mustard, remaining salt in small bowl.
Serve steak with potatoes and sauce. Garnish with remaining scallions.