Place tomato ketchup, mayonnaise, horseradish sauce, chopped onion, chopped dill, lemon juice, worcester sauce in a bowl.
Combine all the ingredients in a bowl and whisk well. Adjust seasoning with salt if needed.
Cut off the ends from the prawns. Keep the prawns and trimmings aside for later.
Cut the grapefruit in half. Use a spoon and scoop out the flesh without breaking through the skin.
Lay the lettuce hearts around the grapefruit cup and fill the inside with the prawn cocktail .