Rinse and dry the squid tubes, score and slice into pieces. Place in a bowl together with the prawns, season with salt and pepper, and coat with the cornstarch.
Heat the pan until very hot, add 2 tsp oil and swirl around to coat the side. Stir-fry the prawns and squid over high heat until prawns changed colour and squids curled up. Remove from the pan.
Reheat the pan, add 2 tsp oil and stir-fry the scallops until just cooked. Do not overcook. Remove from the pan.
Reheat the pan, add 1 tbsp oil and stir-fry the onion, garlic and ginger over medium-high heat for 3-4 minutes until just softened. Increase the heat to high and toss constantly for 3-4 minutes. Add the mirin and 1/2 tsp salt to the pan with some cracked pepper and toss well.
Return all the seafood to the pan, add little water and toss until well combined and heated through.
Transfer to a serving dish, sprinkle with seaweed flakes. Serve immediately with steamed rice.