Add half of the prawns to a food processor with the lard (or vegetable shortening), spring onions, ginger, sugar, salt, cornflour and sesame oil. Pulse to a paste.
Chop the remaining prawns into roughly 1cm pieces, then add to the food processor and pulse only twice to combine, to create a bit of texture.
Put the white and black sesame seeds in a shallow dish. Spread a quarter of the prawn mixture onto a slice of bread so that the mixture is around 1cm thick. Gently place the bread, prawn side down, into the sesame seeds, so that the seeds coat the prawn mixture all over. Lay the bread, uncovered side down, on a tray lined with baking paper, then repeat with the remaining filling and slices of bread.
When you’re ready to cook the toasts, pour enough neutral oil into a large frying pan to fill it to a depth of about 2cm. To test that the oil is hot enough, add a small piece of bread to the pan; once it begins to sizzle, the oil will have reached the correct temperature.
Fry the prawn toasts in batches for thee to four minutes on each side, then drain on paper towels.
Cut the prawn toasts diagonally into quarters and serve topped with Kewpie mayonnaise, katsuobushi flakes and nori flakes.