Cut down the side of the squid so that it can be laid flat on a chopping board. Using a sharp knife, lightly score the inside flesh in a crisscross pattern, then cut the squid into 3 cm (1¼ inch) squares. Chill until required.
Make the dressing. Put the garlic in a small bowl, add the oil and stir in the parsley. Leave the flavours to infuse for at least 15 minutes.
When ready to serve, heat a ridged griddle pan over a high heat until searing 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 warmed serving platter.
Add the squid flesh (but not the tentacles), the clams and mussels to the pan (discarding any that don't shut when tapped), 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 2–3 minutes more, then transfer everthing to the serving platter and drizzle all the seafood with the remaining dressing. Serve immediately with lemon wedges, avocado dip (see below) and bread, if liked.