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.
Note: Rub can be made and stored in airtight container at room temperature for 1 month.