Trim the fennel. Weigh out 100 g (3½ oz) of the central heart and cut it into large mirepoix dice. Chop the young green leaves from the fennel together with the parsley until you have a fine, moist, green purée, and squeeze in a napkin to extract the juice, which should be set aside in a bowl.
Chop the shallots and soften them in the butter in a small heavy pan