Preheat the oven to 300°F (150°C).

Season the chuck roast evenly on all sides with the seasoned salt.

Heat a large skillet over medium-high heat. Sear the roast for about 3 to 4 minutes per side, until well browned.

Place 2 onion halves in a Dutch oven or roasting pan. Place the seared roast on top of the onions. Pour in the beef stock, making sure the roast is partially submerged. Cover tightly with a lid or foil and roast until the meat is fork tender, about 4 to 6 hours. About 45 minutes before the roast is done, add the carrots, celery, potatoes, and the remaining onions to the pot. Re-cover and continue roasting until the vegetables are tender.

Use tongs to transfer the roast to a cutting board and a slotted spoon to transfer the vegetables to a serving platter. Slice the roast and place the slices on top of the vegetables on the platter. Cover with foil to keep warm.

In a small bowl, dissolve the gravy mix in a bit of cold water. Stir the mixture into the hot braising liquid. Cook over medium heat on the stovetop (or return the pot to the oven uncovered) until the gravy thickens, about 5 to 10 minutes.

Spoon the gravy over the pot roast and vegetables. Serve immediately.