Trim and quarter the fennel and cook it in boiling salted water for about 30 minutes. When it is cooked, squeeze the pieces in a tea towel to extract the water.
Sieve the two hard-boiled egg yolks and the fennel into a large bowl with a mouli-légumes. Purée it thoroughly and scrape any bits which have stuck to the blade into the bowl. Add the mustard, lemon jui