Heat the wine in a large saucepan until simmering, then tip in the mussels, cover and cook for about 5 minutes until the mussels have opened. Remove from the pan, discard any that remain closed and leave to cool. When the mussels are cool, remove the meat from half the shells.
Meanwhile, add the prawns to the pan and cook for 3 minutes or until they turn pink and are just cooked through, then leave to cool.
Add the squid rings and tentacles to the pan and cook for 1–2 minutes until they turn white. Remove with a slotted spoon, reserving the cooking liquid, and leave to cool.
Make the dressing. Place the garlic in a small bowl and season. Add the vinegar, then whisk in the oil. Add a little of the cooled cooking liquid. Toss 2 tablespoons of the dressing together with the cooled prawns, squid, mussel meat and the mussels in their shells, then toss in the tomatoes, cucumber and green pepper. Arrange on a serving plate, scatter over the onion and parsley and drizzle with more dressing to serve.