Put the prawns, garlic, ginger, egg white, sugar and soy in a food processor and blitz to a paste. Stir in the spring onion. Scrape into a bowl, cover and chill for 30 mins.
Brush one side of each piece of bread with sesame oil. Spread the prawn mixture on top, taking it right to the edges of the bread. Brush the beaten egg carefully over the top and sides and sprinkle liberally with sesame seeds so they stick all over.
Heat 2-3cm oil in a sauté pan or deep frying pan until hot, and cook each piece of bread (it’s easier to cook one at a time), unspread-side down for 1½ mins, then carefully turn over and cook for 1-2 mins on the prawn side or until the sesame seeds are golden and the prawn paste cooked through. Cut each piece into four triangles. Serve with soy sauce (or a sweet & sour sauce) for dipping, if you like.