Combine sour cream, mayonnaise, 1 cup watercress, garlic salt, and pepper in a food processor; pulse until well blended. Cut roast beef slices in half; roll loosely. Arrange baguette slices on serving platters. Place 1 roast beef roll on each slice of baguette. Drop 1 teaspoonful of sour cream mixture over roast beef. Garnish with a sprig of watercress.