Put the shellfish and 100 ml (3½ fl oz) water in a large saucepan with a tight-fitting lid. Cover and bring to the boil over a high heat. Cook, shaking the pan frequently, for 4–5 minutes, or until the shells have opened. Remove with a slotted spoon, discarding any that remain closed. Remove the flesh from the shells and put in a large bowl.
Return the pan with the cooking liquid to a high heat, add 400 ml (14 fl oz) water, the onion and bay leaf and bring to the boil. Add the seafood and cook for 2–3 minutes until the prawns turn pink and the scallops are opaque all the way though. Remove with a slotted spoon and add to the shellfish.
Boil the liquid until reduced to 50 ml (2 fl oz). Remove from the heat and stir in the lemon juice, garlic and oil. Season with salt and pepper, pour over the seafood and shellfish and toss well.
Leave to cool to room temperature, then toss in the remaining ingredients. Cover and chill for at least 30 minutes and up to 24 hours. Serve the salad with crusty bread.