Using a crockpot, add roast, packaged mixes, water and onion.  Cook on low for 6 to 8 hours.  Three hours before serving, add the potatoes and continue cooking. 

I know the package mixes seem  little odd, but they give the meat and potatoes a wonderful flavor.  I enjoy carrots with my pot roast, so I usually throw in some cut up or baby carrots at the same time I throw in the potatoes.  After I remove the roast from the liquid, I use a spoon with a strainer to take out all the other good stuff and then my husband will take the juice out of the crockpot and put it into a sauce pan with some gravy flour and this makes a excellent sauce to pour over the roast and pottoes.  Biscuits are great to soak up any leftover juices.