Fire up the oven an hour or two before you're ready to cook. When the temperature has reached 180°C, or slightly lower (check with your thermometer), and the smoke and flames have died down, you can start cooking. Always keep an eye on the fire and top up the heat source, if needed. Clean and wash your shellfish, pulling the beards off the mussels and throwing away any open ones. Bash the garlic with a good pinch of salt in a pestle and mortar until creamy, then grate in the lemon zest (reserve the lemons), add a good pinch of pepper and pour in enough oil to make a dressing.
Spread the shellfish out in a large roasting tray and drizzle with the dressing. Toss everything together, then spread the shellfish out again. Halve the lemons, add them to the tray, then slide it into the hot oven and roast for 10 minutes or until all the shells have opened and the prawns and scallops are golden and cooked through. Keep an eye on them and rotate the tray for even cooking. Serve the shellfish straight away, scattered with chopped herbs, and with lots of napkins!