Add 1 tbsp oil to pan, brown beef (in batches if needed) and transfer to the slow cooker once each batch is browned.
In a mixing bowl, combine the onions, golden mushroom soup, soy sauce, mustard, worcestershire sauce, minced garlic, salt and pepper. Pour over the meat in the slow cooker.
Add mushrooms next, but if you're home throughout the day while cooking, I like to add the mushrooms in the last hour. Mix with the meat/sauce.
Cook, covered, on low until meat is tender, 6-8 hours.
When ready to serve, turn off slow cooker and mix in the sour cream.
Serve with cooked egg noodles (or rice) and parsley if desired (I desire parsley!)