Break off the stalk, pull off the tough outer leaves from the base with a small sharp knife. Round the base slightly to neaten. Hold the top in one hand, slice off the bottom third. Discard the top two thirds - it is mainly fibrous leaf.
Place the hearts in cold water with the juice and skin of half a lemon to stop discoloration.
When ready, place in boiling water well-salted water (with a little lemon juice). Weigh down with a plate.Simmer for 15-20 minutes.
Pierce the centre of the heart to check it is tender.
Lift out into iced cold water. Drain and scoop out the choke. Fill the centre with, e.g. white crabmeat mixed with mayonnaise and chervil.