Preheat oven to 180°C. Season the beef with salt and pepper. Add a little oil to the beef strips, mix well. Heat a wok or frypan, ensure it is hot. Stir-fry the beef in 3 batches, remove each batch and place in a casserole
dish. Add a little oil to pan, add onion, garlic and mushrooms and cook for 2 minutes or until onions softens. Add the tomato paste and stock, stir until the mixture boils. Pour over the beef in the casserole dish and stir to combine. Cover the casserole dish, place in oven, cook for 30 - 40 minutes or until beef is tender. In the last 10 minutes of cooking time, stir in the combined sour cream, mustard and cornflour. Serve with pasta or egg noodles and sprinkle with chopped dill or chives.