Pour the oil into a heavy bottomed pan or a flameproof casserole and put on a low to medium heat. Add the parsley, lemon zest and chilli flakes and grate or crush in the garlic. Sizzle briefly but be careful not to burn.
Increase the heat and add the mushrooms. Stir and fry briefly then sprinkle with the salt and stir again. Cover with a lid then turn the heat down to low and leave to simmer for 10 minutes.
Add the lemon juice and stir again, then recover and cook for another 10 minutes, checking and stirring once or twice.
Scatter a little more parsley over the mushrooms and serve.