Cut two slices of bread and butter both sides of both slices.
In a dry pan on high heat, sear both sides until they turn a beautiful golden brown.
On the inside of one of those slices of bread, spread a thick layer of the mayonnaise.
On the other, spread a thin layer of mustard. Or a thick layer, depending on your mustard tolerance.
Finely, finely slice some lettuce and chop it up a little, then add it to the bottom piece of bread.
You can, if you like, season and dress the lettuce. I like to use a little salt and lemon juice.
Spoon the caramelised onion over the top of the lettuce.
Make a few very fine slices of beef, ideally somewhere in the middle, where the beef is still nice and pink. Fold them down gently into the sandwich. Try to keep them fairly loose.
Season the beef if necessary with a little salt and optionally a little pepper.
Put the sandwich together and slice it down the centre.