Adjust an oven rack to the middle position and heat the oven to 450 degrees. Cut eight 12-inch sheets of aluminum foil.
Combine the oil, shallot, tarragon, 1/4 teaspoon salt, and 1/8 teaspoon pepper in a bowl. Toss half of this mixture with the oranges in a bowl and toss the remaining mixture with the carrots and fennel in a separate bowl. Pat the chicken dry with paper towels and season with salt and pepper.
Following the photos, arrange the carrot-fennel mixture in the center of four pieces of the foil. Lay the chicken over the vegetables then spoon the orange mixture over the top.
Place the remaining pieces of foil on top and fold the edges over several times to seal. Place the packets on a rimmed baking sheet and cook until the chicken registers 160 to 165 degrees on an instant-read thermometer, about 25 minutes. (To test the doneness of the chicken, you will need to open one of the packets.)
Carefully open the packets, allowing the steam to escape away from you, and let cool briefly. Smooth out the edges of the foil and, using a spatula, gently slide the chicken, vegetables, and any accumulated juice onto individual plates. Sprinkle with the scallions and serve.
Adjust oven rack to lower-middle position and heat oven to 300 degrees. Combine rub ingredients in small bowl.
Pat roast dry with paper towels. Separate roast into 2 pieces along natural seam and trim fat to ¼-inch thickness. Tie kitchen twine around each roast at 1-inch intervals.
Crisscross two 30 by 18-inch sheets of heavy-duty foil inside large roasting pan. Place onions, potatoes, carrots, and bay leaves in center of foil and drizzle with soy. Set roasts on top of vegetables. Rub roasts all over with rub. Fold opposite corners of foil toward each other and crimp edges tightly to seal. Transfer pan to oven and cook until meat is completely tender, about 4½ hours.
Remove roasts from foil pouch and place on carving board. Tent meat with foil and let rest 20 minutes. Discard onions and bay leaves. Using slotted spoon, place carrots and potatoes on serving platter. Strain contents of roasting pan through fine-mesh strainer into fat separator. Let liquid settle, then pour defatted pan juices into serving bowl.
Remove kitchen twine from roasts. Slice roasts thinly against grain and transfer to platter with vegetables. Pour ½ cup pan juices over meat. Serve with remaining pan juices.
The foil packets can be assembled and refrigerated for up to 3 hours before baking. Bake as directed, increasing the cooking time to about 30 minutes.Rub can be made and stored in airtight container at room temperature for 1 month.