Place the loaf on its side on a board, then carefully slice off the top and put to one side. Upright the loaf again. Place a knife about 2.5 cm from the outside and cut into the bread down all 4 sides. Pull the bread from inside.
To make the mayonnaise, put the egg yolks, mustard, sugar and vinegar in a high-sided bowl. Whizz with a stick blender while slowly dribbling in the oils toget