Rub duck legs with 1 tablespoon salt, and place in a bowl. Sprinkle with pepper and remaining 2 tablespoons salt. Add thyme and bay leaves. Refrigerate covered, turning duck occasionally, for 10 to 12 hours, depending on how strong a salt cure is desired.
Preheat the oven to 300 degrees F.
Rinse duck under cool running water, and pat dry. In a medium skillet over medium heat, place duck skin side down; cook until fat renders and skin browns, 15 to 20 minutes.
Transfer meat and reduced fat to a small casserole, placing duck skin side up in 1 layer. Add enough lard to cover. Cover casserole with a lid, and place it in the oven. Cook until duck has rendered all of its fat and the meat it is falling from the bone, 2 to 2 1/2 hours.
To preserve duck, pour a layer of fat from the casserole into the bottom of a small terrine. Pack duck on top, and strain the remaining fat from the casserole over the meat. Add additional melted lard, if necessary to cover. Be sure there are no air bubbles in the fat, or the duck will not be properly preserved. Cover, and refrigerate for at least 1 week to allow flavors to mellow.
Preheat the oven to 400 degrees F.
Place terrine in warm water to melt fat. Remove duck from fat; wiping off excess fat and reserve it for other uses (it makes a wonderful substitute for butter in savory dishes). Place duck in a shallow baking dish, and bake for 5 minutes. Pour off fat, and add to reserved fat. Continue baking until duck is very hot and the skin is crisp, 10 to 15 minutes more.
Divide greens between 4 plates. Drizzle with vinaigrette, and top each bed of greens with a crispy duck leg.
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