Sandwiches1red onion, halved and thinly sliced2 tbspwhite wine vinegar2small sourdough ficelles, halved, or 1 large sourdough baguette, cut into 4 pieces12butter lettuce leaves20cooked medium tiger prawns, peeled and cleanedTo serve:lemon wedgesTomato chilli mayonnaise1egg yolk1 tspDijon mustard2/3 cuplight olive oil1egg tomato, peeled, seeded and finely chopped2 tsplemon juice1fresh red birdseye chilli, finely chopped
Combine onion and 1 tsp sea salt in a bowl and mix well, then leave for 20 minutes. Meanwhile, for tomato chilli mayonnaise, process egg yolk and mustard in a small food processor until combined, then, with motor running, add olive oil in a steady stream until thick and emulsified. Add tomato, lemon juice and chilli and process until smooth and combined. Transfer mixture to a small bowl and season to taste.
Rinse red onion under cold water, then drain and pat dry on absorbent paper. Combine onion and vinegar in a bowl, season to taste with freshly ground black pepper and mix well.
Cut bread in half horizontally and fill each piece with 3 lettuce leaves and 5 prawns. Top with a little marinated onion, drizzle with tomato chilli mayonnaise and season to taste with a little freshly ground black pepper.