First, make the pesto. Put the walnuts, chopped coriander, garlic, Parmesan, chilli and olive oil into a blender and blitz to make a smooth mixture. Season with salt.
Put the prawns in a food processor with the oil and egg whites and season with salt and pepper. Blitz until smooth.
Beat the whole eggs with the milk in a bowl and season with salt, pepper and a grating of nutmeg, then set aside.
Divide the prawn mixture between 4 of the slices of bread, spreading it evenly, then top with the remaining slices. Cut the sandwiches in half, then dip each half into the egg mixture.
Pour some oil into a frying pan to a depth of about 1cm and heat. Shallow-fry the sandwiches, a few halves at a time, turning them until golden on all sides – this will take about 4 minutes in total.
Drain the sandwiches on kitchen paper, then keep them warm in a low oven while you cook the rest.
Cut the sandwiches in half or into bite-sized pieces. Serve warm with the pesto and sprinkle a few chilli flakes on top if you want your sandwich to have a bit of a kick.
