Stir together sour cream and horseradish in a small bowl. To assemble, spread a tortilla evenly with 1 heaping tablespoon of the horseradish mixture. Cover with watercress and a quarter of the roast beef, making sure the ingredients don't quite touch the edges. Spread another heaping tablespoon of the horseradish mixture over the top, followed by a quarter of the apple or pear slices, and 1 tablespoon of blue cheese. Roll up the tortilla. Complete the wraps with the remaining ingredients. Cut in half on the bias. 4 servings.