In a large bowl, combine the orange juice, vinegar, marmalade, sugar, pepper and salt; stir until sugar is dissolved.
In another large bowl, combine the grapefruit, oranges, asparagus, cucumber, onion and avocado. Add the marinade and toss to coat. Cover and refrigerate for 2 hours.
Drain and reserve marinade. Arrange lettuce on plates; top with fruit mixture. Sprinkle with mint if desired. Serve with reserved marinade.
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