Bigarde sauce is a classic French sauce to accompany roast duck or duckling.
It is made by preparing a stock of sugar cooked to a pale caramel with wine vinegar, which is then mixed with brown veal gravy. This stock is then used to dilute the juices in the roasting pan, and is cooked over a strong heat. When it is just finished, it is finished with the juice of an orange, a bit of lemon juice, and strained through a fine cloth. It is finished with very fine julienne of orange peel which has been blanched and cooled in cold water. The sauce can be flavored at the last moment with a very small amount of curaçao.
When ducks and ducklings are served à la bigarde, they are garnished with quarters of peeled and seeded oranges, which are surrounded with a border of half slices of oranges, cut with fluted-edge cutters.
Larousse (1961) includes an older recipe from Carême's L'art de la cuisine française au XIXe siècle, which calls for carefully peeling a bitter orange and slicing it into fine shreds, which are then boiled in water for a few minutes, drained, and added to a saucepan of espagnole sauce, a bit of game glaze, a pinch of mignonette, and the juice of half a bitter orange. Once it is brought the boil it is finished with a piece of butter.