To make the hollandaise sauce, place the egg yolks in a heatproof glass bowl.
Heat the butter in a saucepan until foaming, then pour gradually onto the egg yolks, whisking all the time. Add the lemon juice to taste, then pour into a heatproof measuring cup. Half-fill a saucepan with hot water from the kettle and place the cup of hollandaise in the saucepan to keep warm. When the water cools, put the saucepan on gentle heat, but do not let the water boil too long or the sauce will scramble.
Keep the sauce warm while you are waiting to serve it; it will sit quite happily like this for a couple of hours. Half-fill another saucepan with water, add a good pinch of salt and bring to the boil. While it is heating up, remove the woody ends from the asparagus spears by snapping off about 1 1/2 inches of the bottom of the stalk. Discard the woody ends.
Cook the asparagus by dropping them into the boiling water, and bringing back up to boil. Remove the lid and boil, uncovered, for another 4–7 minutes until just cooked. While the asparagus is cooking, toast the bread and butter it. Remove the asparagus and place on the buttered toast. Drizzle with some of the hollandaise sauce and serve the rest in a pitcher on the table.