Cut the sandwich bread into equal squares to have 4 pieces.
Spread mayonnaise on one side of two slices of bread. Spread GIA onion puree on one side of one slice of bread.
Put the shrimps and lettuce leaves on the slices, at this point put the slice with the onion paste on top of the one with the mayonnaise, and again one with the mayonnaise on top of the previous one, until you get three layers.
Now cover with another slice of bread and cut the sandwich in half to obtain a triangular shape.