2) In a shallow dish, whisk together the grapefruit juice, ketchup, mustard, sugar, chili powder, cumin, olive oil and liquid smoke. Add the prawns and toss to coat.
3) Place on the griddle and cook 2 minutes per side, or until the prawns are bright pink and cooked through. Serve half the prawns over the rice and top with scallions. Reserve the remaining prawns for another dish.