I do add thick sliced onions on the bottom...
For pot roast, I like the less sweet Spanish Onions...
I use a very large cast iron dutch oven, and for the last hour & a half usually add white eastern potatos, in chunks, and carrots, cut in one inch chunks...
I have added Onion soup mix... sometimes...
have also found (I know it sounds weird) about a tsp of fresh ground nutmeg is really tasty!
I have never used canned soup in a pot roast... but it might be good also...
I use Cambell's soup mushroom, aspargrass, etc. in a lot of recipes... but not yet for pot roast...
I try to not use condensed soup, if I can avoid it, as the salt content is just so high...