I would probably put a cup of water in the bottom of the crock pot to prevent burning to the bottom. With that said, you can do the same thing in the oven. Place a large sheet of tin foil in a roasting pan. Lay the pot roast in, top with the can of mushroom soup and then the onion soup mix. (I use 1 can of reg. mushroom soup and 1 of Golden Mushroom soup with a full package of onion soup, also add a can of mushroom pieces or buttons, a bag of frozen pearl onions, and a bag of fresh baby carrots) seal up the top and sides of the tin foil and bake at 325 degrees for 4 hours.
When done, if the "gravy's not thick enough, dump it into a pot (be sure to scrap the bottom of the foil for all the "good stuff")and mix a little cornstarch or flour with water and stir in over medium heat.
I often put it all together and set the oven on a programmed start. Come home to a GREAT meal.
This is my family's FAVORITE meal. Serve over noodles.