Cut the hard-boiled eggs in half and separate the yolks. Place the yolks in a bowl and set them aside.
To the yolks, add the mayonnaise, milk, salt, and black pepper. Mix well until a paste forms.
Chop the egg whites into small pieces and add them to the yolk mixture. Also, add the chopped spring onions. Mix everything well to combine. Your filling is now ready.
Remove the crusts from the bread slices and spread a little butter on each slice.
Take a generous scoop of the filling and spread it evenly on one slice of bread. Place another buttered bread slice on top.
Slice the sandwich in half and serve.