Lemon Mayonnaise: Whisk egg yolks, salt and mustard together in a heavy bowl.
Start adding the grapeseed oil drop by drop while you continue to whisk. As the mixture thickens add the oil in a steady stream.
Continue to whisk in the olive oil and add lemon juice to taste. If the mayonnaise thickens too much thin it with a little hot water.
Adjust lemon juice and seasoning. Keep refrigerated and use within a few days.
Crayfish Sandwich: Melt butter in frypan and sauté garlic.
Add crayfish and cook until the meat whitens then add the diced parsley and remove frypan from heat.
Make up two large sandwiches with the sliced bread with generous dollops of lemon mayonnaise, a couple of sprigs of watercress, mashed avocado and the still warm crayfish meat.