Put the prawns, garlic, ginger, sugar, salt, egg white and sesame oil in a food processor and process until smooth. Put the mixture in a bowl and stir in the spring onions.
Spread about 2 tablespoons of the prawn mixture to the edges of each piece of bread and cut each in half diagonally. Brush the tops and sides of each piece with the beaten egg, then press into the breadcrumbs to coat.
Heat the oil in a wok or deep-fryer until smoking (180°C/350°F), and deep-fry the toasts in batches until golden on both sides, about 2 minutes. Remove and drain on paper towel. Serve hot with a sweet and sour dipping sauce.