Bring a pan of water (perhaps with a few of the asparagus trimmings) and plenty of salt to the boil. Trim 0.5 cm off the ends and peel the white asparagus thoroughly! Cook the asparagus whole for 5–8 minutes, depending on their thickness. Test with a knife or cut off a piece – they should be just tender.
Melt the butter and leave it to stand for a moment so that it separates into clarified butter and buttermilk at the bottom. Bring the lemon juice and water to the boil and let it boil for 1 minute. Leave it to cool slightly. Whisk the egg yolks in a saucepan, together with a little salt and the lemon juice and water, over a low heat or in a bain-marie, whisking thoroughly until the mixture becomes light and airy. Gradually add the warm clarified butter, first a little at a time, then in a steady stream, until it has emulsified into the egg mixture. Season to taste with lemon juice, salt and pepper.
(If the sauce separates, add a tablespoon of ice-cold water to the edge of the pan and whisk vigorously and concentrated at the edge with the water to bring the sauce back together).
Serve the warm asparagus with the hollandaise sauce poured over the lower third. Tip: you can also enhance the hollandaise with finely chopped herbs such as chervil, chives and/or basil.
