Heat the oil in a frying pan over a medium heat, add the onion, chilli and ginger and fry until the onion is soft. Add the garlic and fry for a further minute. Set aside to cool.
Place the cooled onion mixture, prawns and pork in a food processor and blend until a paste is formed. Add the egg, fish sauce, coriander, lime rind and a little salt and pepper and blend once more.
Spread this mixture over the bread in a layer about 1 cm (½ inch) thick. Sprinkle with the sesame seeds and cut into neat triangles.
Pour the oil into a deep-fat fryer or large saucepan to a depth of at least 7 cm (3 inches) and heat to 180–190°C (350-375°F), or until a cube of bread browns in 30 seconds. Fry the prawn toasts in batches of 4 triangles at a time, prawn-side down first, for 3 minutes, then turn over and cook on the bread side for a further minute. The prawn toasts should be golden brown. Drain on kitchen paper and keep warm while you cook the remainder. Serve with lime slices to squeeze over the toasts, garnished with coriander sprigs.