The day before you want to serve the crêpes, sear the duck legs in a hot heavy-bottomed saucepan, browning them as evenly as possible all over. Remove and set aside.
Lower the heat to medium and add the onions, cooking them until they are transparent, about 5 minutes.
Add the duck legs and whiskey, cover, and simmer for 1 hour. Remove from the heat, cool, and refrigerate overnig