Steam the asparagus over plenty of boiling water for 3-5 minutes (depending on the thickness of the spears).
Bring a pan of salted water to the boil and add the vinegar. Whisk to make a whirlpool. Once it's settled, crack an egg in the middle. Simmer for 2-3 minutes, remove and keep warm. Repeat with the other eggs.
Make the hollandaise by putting the egg yolk, lemon juice and water into a blender. Turn on the machine and then slowly pour the warmed butter through the lid. Season with cayenne pepper and salt.
Pile 5-6 spears of asparagus on to each toasted brioche. Top with a poached egg. Spoon on the hollandaise sauce and serve.