Prepare the MarinadeMix the soy and Sherry together in a baking dish large enough to hold the duck pieces. Using a sharp knife, make diagonal cuts into the duck breasts about 1 inch apart in the duck skin, but do not cut into the meat. Place the duck, skin side up, into the marinade; cover with plastic wrap and refrigerate at least 2 hours, and up to six. (Conversely, you can put all of this into a zip-top bag, remove as much air as possible and seal; place the bag into a baking dish, in case of leaks - this is my preferred method to marinate anything.)
Prepare the Confit Leg and Thigh: Heat the oven to 400°F. After at least the minimum marinating time, remove the leg/thigh combo and place in a baking dish that just holds them snugly. Cover with foil and place in the oven; bake for 1 hour, remove the foil and bake 1 hour more. (Leave the breasts in the marinade until ½ hour before you are ready to cook them. Remove them and bring to room temperature.) Meanwhile, bring the cherries, chicken stock, beef or duck stock, port and thyme sprig to a boil in a saucepan. Simmer until reduced to 1 cup. , can be quick on high heat, or slower on a slower heat. I like to do slower to allow the flavors to develop.
Heat a large skillet over medium-high heat; pat the duck breast dry then place, skin side down, into the skillet. Cook until the skin is crispy, about 10 minutes; turn and continue to cook to your desired degree of doneness (duck breasts should be cooked medium to rare; do not overcook or it will be tough.)
Remove the thyme sprig from the port/cherry sauce. Stir in the cornstarch mixture; then using a whisk, whisk in the butter, a piece at a time. Taste and adjust seasonings with salt and pepper (in honor of Julia, let's use white pepper).
Place a let/thigh on each plate. Slice the breast and fan out. Spoon the port/cherry sauce over and serve.