Zimna golonka z pietruszką, lekką galaretką pietruszkową i bukietem przypraw

Filter then finely before reducing: it’s a gesture that gives a cleaner and cleaner frosting.
At what point should I add the parsley to the jelly?

Add the parsley when the reduced juice is lukewarm: neither hot (colour that fades, fragrance that evaporates), nor already too cold (risk of seizing before assembly).

If you prefer, you can also reserve the majority to parse at the demolition in order to keep a very fresh note.
How long before carving the cold parsley ham?

Let soak in the refrigerator for at least