Preheat the oven to 275F. Start by rubbing the mustard all over the chuck roast.
Combine the spice rub ingredients and rub it over both sides of the roast. Place it in a baking pan and cover with foil.
Bake for 2.5-3 hours, until the roast is super soft.
Remove it from the oven and let it cool slightly.
Raise the oven temperature to 425F.
Cut the roast into cubes and sprinkle with brown sugar. Pour the BBQ sauce over everything, and toss well to coat.
Bake the cubed meat for 15-25 minutes, until it's caramelized.