Place the optional carrots and potatoes in the bottom of the crock pot. Cover with half of the sliced onion. Evenly sprinkle the dry soup mix over all sides of the pot roast. Place the roast in the crock pot and top with the remaining sliced onions. Cover the crock pot and cook on low (or "warm" if your crock pot has it) for 8-10 hours. You want it on the lowest setting possible. Do not add any water and do not lift the lid during the first 8 hours. The crock pot will create steam to keep the meat moist. Lifting the lid will release that steam and dry out the roast. When the time is up, remove the roast to a cutting board and remove the potatoes and carrots with a slotted spoon to a platter. Slice the roast just before serving. Skim off any fat from the juices in the crock pot or strain the liquid if desired. Serve alongside the roast and vegetables, or thicken it with a cornstarch slurry.