Chop the onion finely and sweat it, very gently, in the butter without letting it colour. Leave to cool.
When the onion is cool, tip it onto the mince in a large bowl. Add a grating of nutmeg, a good grinding of black pepper and plenty (about a flat teaspoonful) of sea salt.
Break the egg into a mug and mix with a fork. Then add to the meat and onion mixture and mix it all thoroughly together – the only satisfactory way to do this is with your hands.
Divide the mixture into five (or six) equal sized pieces. Flour the work surface and your hands and make the first piece into a smooth ball.
Flatten it on the floured surface and, using your floury hands, turn it round and round until it is an even ‘burger’ shape (as thick or thin as you choose though, if you make it too thin, it will have a tendency to break).
Turn over and repeat the turning round and round.
When the burger is a nice smooth shape, remove to a plate with a fish slice.
Repeat the exercise with all the other pieces and, if there is time, keep the finished burgers in the refrigerator for half an hour to firm up before you cook them.