Fill a pot with just enough water to cover the vegetables.
Add the dill and season with salt, pepper and a pinch of sugar.
Boil the vegetables until they are al dente – not too soft.
Brown the butter in a pan until golden brown.
Strain off the salt through a sieve into a bowl in an ice bath.
Whip the butter until it is smooth, then blend in the onion and the dill.
Season with salt, pepper and a little lemon juice.