||Coat the beef strips in a little oil. Season with salt and pepper. Preheat a non-stick pan to moderately-hot. Keep the heat at moderately-high.
||Cook the beef in 3 batches. Cook the first side until the first sign of moisture appears on the uncooked side, turn and cook other side of beef strips. Remove each batch and set it aside.
||Add butter to the pan, add the onion and cook until soft. Add the mushrooms, cook until tender. Add beef stock and tomato paste, stir to combine and bring to the boil. Reduce heat and simmer for 2 minutes.
||Stir in the combined sour cream combined and cornflour. Stir until well combined, simmer for 1 minute. Return beef to pan and simmer for 2 minutes.
||Serve with egg noodles