Heat oil in a large pan over HIGH heat. Brown beef and onion (You may need to do this in batches) then add to slow cooker.
Add mushrooms, tomato and recipe base to the slow cooker and stir through. Cover and cook for 4 hours on HIGH or 8 hours on LOW (Keep covered during cooking. Cooking times may vary between slow cooker models, we used a 5.5 L appliance).
Before serving stir through sour cream and serve with pasta and parsley.
NO slow cooker, NO problem, place prepared casserole in an oven proof dish, cover and cook at 160°C (140°C fan-forced) for 3 hrs or until tender.