Place the duck breasts in the insert of a 5 1/2 to 6 quart slow cooker and add the fat, garlic, shallot, salt, pepper, and herbes de Provence. Cover, set the slow cooker on HIGH, and cook for 1 hour. Reduce the heat to LOW and cook until the duck is very, very tender--at least 4 and up to 8 hours. Use a slotted spoon to remove the duck from the fat, which can be strained, frozen, and reused. Serve the duck hot or at room temperature, or use it to make cassoulet.
NOTE: You can buy canned goose fat in gourmet food stores. If you have not accumulated your own duck or goose fat, chicken fat is perfectly acceptable.
There are three ways to get chicken fat for this recipe: You can skim congealed fat off the top of chicken soup or stock, buy it in a plastic tub from a kosher butcher, or make it yourself. To make your own, place raw chicken fat in a heavy saucepan and cook very slowly over medium-low heat until the fat has melted, the connective tissue has darkened and crisped, and any water has evaporated. Strain the rendered fat into a bowl. Rendered chicken fat can be kept in a sealed container in the freezer for up to 6 months.
If you cannot find herbes de Provence, you can make your own by combining equal amounts of dried tarragon, rosemary, chervil, basil, and thyme leaves.
Recipe courtesy of Lora Brody's Slow Cooker Cooking (William Morrow, 2001)