Heat the oil in a large pan and add the onion. Cook over a low heat for 5 minutes until softened. Add all the mushrooms and increase the heat to medium. Cook for a further 5 minutes, adding the chopped garlic for the final minute.
Stir in the cooked roast beef along with the crème fraîche and chicken stock. Reduce the heat and simmer for 2 minutes.
Meanwhile, cook the microwave rice to pack instructions. Divide between 4 plates and serve the stroganoff on top, garnished with the chopped parsley.