Cut the dry ends of the asparagus (∼½ inch / 1cm). Peel the white asparagus entirely (except the heads) and the green ones only at the ends.
Wash them and place on a steamer tray. Steam for 30 minutes at 96°C/205°F.
Sauce Hollandaise
Melt the butter and set aside.
Set up a water bath and place a large bowl on top. Add the egg yolk, mustard, and lemon juice in the bowl and mix until fluffy. The water bath should not be too hot. Otherwise, the eggs will get cooked, and you don't want that for your sauce hollandaise.
Slowly pour in the melted butter and stir constantly. When the sauce starts to divide into two parts, stop to add more butter, mix until well combined, and then go ahead with adding the butter.
Add two ice cubes and mix until your sauce is smooth and creamy.
Season with one tablespoon white vinegar, salt, and pepper.
Serve your steamed asparagus immediately with the sauce hollandaise.