Dredge the meat chunks lightly in flour and stir in pan over medium heat until browned, but not cooked all the way through. Season with salt and pepper. Remove the meat from the pan, and drain on a paper towel. Move the meat into large crock pot, and add the onion soup. Cook on High for about four hours, then reduce temperature to Low.
If you are using fresh mushroom, add to pot at the midpoint of cooking time. If using canned mushrooms, add 1 hour before serving.
About 15 minutes before serving, stir sour cream and 3-4 tablespoons of flour together. Mix well. Whisk the sour cream a little at a time into the pot and allow to cook another 15 minutes.