Cut 1 ounce (2 slices) deli roast beef slices (rare) into 1/3-inch-wide strips. Cut 1 scallion into 6 long strips. Wrap each of 6 breadsticks with roast beef strips and 1 scallion slice. Roll grissini in 1/4 teaspoon grated fresh black pepper. Combine 3 tablespoons reduced-fat sour cream and 3/4 teaspoon horseradish sauce, stirring well; serve sauce alongside grissini.