Coat the beef strips in a little oil. Season with salt and pepper. Preheat a non-stick pan to a moderate heat. Cook the beef in 3 batches. Cook one side until the fi rst sign of moisture appears on the uncooked side, then turn and cook the other side. Remove each batch and set aside.
Melt butter in 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 and cornfl our. Stir well, simmer for 1 minute. Return beef to pan and simmer for 2 minutes. Serve with noodles or pasta, and either salad or seasonal vegetables.