Cut the squid bodies in half lengthways, reserving the tentacles, open out and pat dry with kitchen paper. Lay them on a chopping board, shiny-side down, and, using a sharp knife, lightly score a fine diamond pattern on the flesh, then cut the squid into 3 cm (1¼ inch) squares. Chill with the squid tentacles until required.
Make the dressing. Put the garlic in a small bowl, add the oil and stir in the parsley. Leave to infuse for at least 15 minutes.
When ready to serve, heat a griddle pan over a high heat until smoking hot. Lightly brush the prawns and squid with half the dressing. Add the prawns to the pan and cook for 3–4 minutes on each side or until they turn pink. Transfer to a warm serving plate.
Add the squares of squid, scored-side down, the clams and mussels to the pan and cook for 5–7 minutes or until the squid turns white and is charred and the clams and mussels have opened. Discard any that remain closed. Add the squid tentacles and cook for a further 2–3 minutes, then transfer everything to the serving plate and drizzle all the seafood with the remaining dressing. Serve immediately with lemon wedges, Avocado Dip (see below) and bread, if liked.