Cut the
grapefruits crosswise at the top. Scoop and chop the flesh removing fibres. Peel the
tomatoes and dice. Peel the
avocado, crush and soak with some
lemon juice. Peel the prawns. Make the sauce with the remainder
lemon juice, yoghurt, 2 tablespoons ketchup and season to taste. Mix the prawns with the
tomatoes,
avocado and the
grapefruit. Blend with the sauce and pour into the scooped
grapefruit. Serve cold.