Peel the prawns and leave the tails attached; put the heads and shells into a pan with the stock, bring to the boil and simmer for 15 minutes; strain the stock into another pan and discard the shells; keep the stock on a low heat
Meanwhile, heat 1 tablespoon each of the oil and butter in a wide pan with a lid over a medium heat; soften the onions, uncovered, then stir in two of the garlic cloves and cook for 2 minutes before turning up the heat slightly and adding the rice; stir until the grains are well coated with oil and beginning to turn translucent, then pour in half the wine and stir until the rice has absorbed most of it, then repeat with the rest
Once all the wine is absorbed, add a ladleful of stock and stir until absorbed, then repeat until the rice is tender and the dish has a loose, creamy consistency – about 30 minutes; you probably won’t need all the stock
Beat in the remaining butter and season to taste; cover and leave for 5 minutes to rest; meanwhile heat the remaining oil in a small pan and fry the last garlic clove for a minute; add the prawns and squid rings and cook for 3-4 minutes until the prawns are pink and the squid is tender; add the mussels and clams and cook for a few minutes, then spoon over the risotto; sprinkle with the parsley and serve
6 servings