Pat the prawns dry and sprinkle with salt and pepper. Let sit for 20 minutes.
Meanwhile, make the sauce by mixing together the soup, milk, water, cornflour slurry and melted butter.
Preheat oven to 245 C / Gas 9.
Heat a large frying pan with 1 tablespoon oil over high heat. Stir-fry the prawns until cooked through, 2-3 minutes. Set aside
Heat the same frying pan with 1 tablespoon oil over high heat. Stir-fry the onion and salt until soft, 3-4 minutes. Add the prawns, seafood sticks and vegetables and mix well.
Add the sauce, stir to mix well.
Spread half of the seafood mixture on the bottom of a baking dish. Cover with 3 lasagne sheets. Top half of the grated mozzarella and half of the grated Cheddar. Repeat this procedure, finishing with cheese.
Bake for 15 minutes or until the top is golden. Sprinkle with basil and serve.