Bring 2 pints of water to the boil in a stainless steel saucepan, add the leeks and cook for 3 minutes over a gentle heat. Remove with a slotted spoon and leave to cool. Bring the water back to the boil, add the asparagus and cook for 3 minutes. Remove with a slotted spoon and drain well. When the leeks are cool enough to handle, squeeze them lightly to extract any remaining liquid. Place in a shallow dish with the asparagus.
For the gribiche, mix together the oil, vinegar and mustard. Add the eggs, capers, truffle and herbs and season with the sugar, salt and pepper. Spoon the gribiche over the leeks and asparagus and leave to cool to room temperature before serving.