This is something my best friends mom used to make for us when she would visit, it is total comfort food. The sour cream is really up to you, my husband doesn't put any on his, but I love the extra creaminess it lends to the sauce.
Spread the onion soup mix on the bottom of the crock pot.
Brown all sides of the pot roast in a skillet, I use about 2 tbs of oil.
Remove pot roast and place in crockpot.
Add soup and flour to the hot skillet, cook until smooth (add water if you are using condensed soup). Pour over the roast in the crockpot.
Add tomatoes and mushrooms.
I usually have this on for about 9 hours on low while I'm at work.
At this point the meat is pretty fall apart tender, I remove the roast and stir the sauce which is pretty thin, you can pour it into a saucepan and reduce it, but it usually thickens up a bit with a few dollops of sour cream I serve this with egg noodles tossed with butter and a few shakes of dill, top with a few spoonfuls of sour cream and then some of the beef.
This should be so tender you can eat it with a spoon.
This doesn't look pretty by any means, but it is heavenly comfort food!