Combine low-fat yogurt and horseradish, stirring with a whisk.
Arrange 3/4 cup Basic Grilled Flank Steak on each flatbread. Top each flatbread with 1/4 cup lettuce, 1/4 cup tomatoes, and 1/4 cup cucumber. Spoon 2 tablespoons yogurt mixture over each serving; roll up. Wrap in aluminum foil or wax paper; chill.