Trim fat from steak. Use your fingers to press pepper onto both sides of steak. In a large skillet cook steak in hot oil over medium heat for 15 to 20 minutes or until desired doneness (145 degrees F for medium rare to 160 degrees F for medium), turning once. Remove from skillet; keep warm.
Add onion to drippings in skillet. (Add more oil, if necessary.) Cook for 5 to 10 minutes or until onion is nearly tender, stirring occasionally. Stir in mustard; remove from heat.
Meanwhile, if using focaccia bread, cut into six wedges; split each wedge in half horizontally. If using bread, toast bread, if desired. To serve, cut steak into bite-size strips. Top bottom halves of focaccia bread or six bread slices with spinach, steak strips, roasted pepper strips, onion mixture, and remaining focaccia bread or bread slices.