Put the onions, carrot, bay leaf, thyme and peppercorns into the milk, bring to the boil and simmer for 3–4 minutes. Remove from the heat and leave to infuse for 10–15 minutes. Strain.
Meanwhile, sauté the sliced mushrooms in a little butter in a hot pan, season with salt and freshly ground pepper and set aside.
Put the fish into a wide pan or frying pan and cover with the flavo