In a small bowl, combine the smoked salt, celery salt, garlic salt and onion powder. If using liquid smoke (see Cook's Note), mix it together with the tomato paste. Rub the tomato paste over the brisket so every side is covered. Then sprinkle the brisket with 1 tablespoon of the salt mixture. Reserve the remaining mixture for future use. Wrap the brisket in foil and refrigerate for at least 6 hours and up to overnight.
Remove the brisket from the refrigerator and let it come to room temperature, about 1 hour.
Preheat the oven to 325 degrees F.
In a large roasting pan or baking dish, toss the onions with the oil to coat. Unwrap the brisket and place it on top of the onions. Cover the entire pan with foil and roast until tender, 3 hours to 3 hours 30 minutes. Using tongs, carefully remove the brisket from the pan and place it on a cutting board to rest for 10 minutes. Thinly slice with a carving knife. Pour the juices and onions over the slices.
If you can't find smoked salt, substitute 2 ounces liquid smoke plus 2 tablespoons kosher salt. To keep the brisket warm, place it in a slow cooker on the warm setting until ready to serve.