Using a sharp knife cut the steak into matchstick thin strips.
Squeeze the juice from 1 1/2 lemons. Slice the remaining 1/2 lemon thinly for decoration.
Heat the oil over a gentle heat and fry the onion slowly for a few minutes, then increase the heat and add the steak to the pan.
Brown and Turn. As soon as the steak is pale brown add the mushrooms, parsley and half of the lemon juice. Stir together well and cook until the mushrooms are done to your taste.
Gradually add the cream and the remaining lemon juice, stirring to heat through.
Serve immediately with rice and a green salad and garnished with the parsley and lemon slices.