Spread 1 side of each flour tortilla with about 2 tbsp. Of the horseradish dip, covering the whole tortilla. Top with 1 or 2 slices of the roast beef. Spread with another 2 tbsp of the horseradish dip. Top with the lettuce leaves. Roll up jelly-roll style, place seam side down on a platter, cover and chill until servings time. Before serving, cut each roll-up crosswise into thirds. Makes 6 servings, 2 roll-ups each.