Preheat oven to 350 degrees F. Split potato rolls apart and brush cut sides with olive oil or butter. Arrange on a baking sheet and toast in oven for 3-4 minutes or until lightly browned.
Combine orange juice, soy sauce, mirin, honey, and ginger in a large zip-top plastic bag; add flank steak. Seal bag and marinate in refrigerator 24 hours, turning occasionally.
Remove steak from bag, and discard marinade. Heat a grill pan over medium-high heat. Coat pan with cooking spray. Add steak to pan; grill 6 minutes on each side or until desired degree of doneness.
Remove steak from pan; let stand 10 minutes. Cut steak diagonally across grain into thin slices; cut slices into 2-inch pieces.
Combine mayonnaise, wasabi paste, and vinegar, stirring well.
Divide sliced steak evenly among crostini. Top each with a dollop of wasabi mayonnaise and garnish with chives, if desired.
Recipe adapted from: www.myrecipes.com.