Preparation Halve cucumber lengthwise, then slice diagonally 1/4 inch thick. Stir together lime juice, water, fish sauce, sugar, red-pepper flakes, and shallot in a small bowl until sugar is dissolved. Arrange pineapple and cucumber on a platter and drizzle with some of dressing. Drape roast beef slices on top and arrange mint alongside. Serve with remaining dressing on the side.
Preparation Halve cucumber lengthwise, then slice diagonally 1/4 inch thick. Stir together lime juice, water, fish sauce, sugar, red-pepper flakes, and shallot in a small bowl until sugar is dissolved. Arrange pineapple and cucumber on a platter and drizzle with some of dressing. Drape roast beef slices on top and arrange mint alongside. Serve with remaining dressing on the side.