Add finely chopped egg whites to the onion.
:- Discard the egg yolks.
Add the mayonnaise, salt and pepper. Mix thoroughly.
Divide the stuffing into 4 portions.
Spread one portion of the stuffing evenly on one slice of bread and cover with another slice. Repeat for the remaining slices.
Toast in a sandwich maker till done.
Serve hot with ketchup.
Variation:
:- If you do not have a sandwich maker you can toast the bread slices and then make sandwich with the filling and press firmly with your hand and serve.
:- If you want to pack it for lunch, then do not toast the bread. Just keep the filling between fresh bread slices and cut off the side crust. Slice diagonally and pack in a box.