For the mayo, put the egg yolk, mustard and a little seasoning into a small blender and whizz briefly. Whizz again and very slowly drizzle in the sunflower and walnut oils until emulsified. Stir through the liquid from the pickled onion jar to loosen. If you don’t have a small blender, this is also made easily in a bowl using an electric whisk. Pour the mayonnaise into a bowl and stir through the pickled onions, cornichons, shallot and ground white pepper. Very gently fold in the chopped boiled egg until just combined.