For the creamy mushrooms: add the butter to a medium pot and fry the mushrooms over high heat, stirring often, until lightly brown. Season with salt & pepper and add the flour. Stir for about 30 seconds, then add the cream. Bring to a boil, then cook for 2-3 minutes until thickened, stirring often. Add the cheese and stir to melt. Remove from the heat. Taste and adjust seasoning if necessary. Set aside, covered, while you prepare the fish.For the fish: Using a non-stick pan, heat the oil. Add the fish skin-side down and fry for about 3-4 minutes until the skin is crisp and golden. Turn the fish over, then fry for just a minute or until just cooked in the middle. Remove from the pan, and serve at once with steamed pak choy and a generous dollop of creamy mushroom sauce.