Heat oven to 160C/140C fan/gas 3. Put the wine, stock, thyme, peppercorns and a pinch of salt in a deep-sided frying pan and bring to just below the boil. Drop in the halibut, turn down the heat and poach for 8 mins or until flaking apart. While the fish is poaching, fry the bacon in a hot frying pan for 2 mins each side until crispy. Remove from the pan, cover with foil and keep warm in the oven.
In the same frying pan, heat the oil and 1 tbsp butter until foaming. Add the mushrooms and cook until you get a lovely golden colour. (If using a king oyster mushroom, turn over to get an even colour on both sides.) Once cooked, add the lemon juice to taste and sprinkle with a little salt.
Lift the cooked fish from the pan, lay on a plate, cover with foil and keep warm in the oven. Turn the heat up on the poaching liquid, reduce by two-thirds, then pour through a fine sieve.
Wash the spinach in cold water and add to the mushrooms. Season, add more butter if needed, and cook for about 3 mins, stirring until wilted. Divide the spinach and mushrooms between two plates and serve the halibut on top. Spoon over the reduced sauce and top with a rasher of crispy bacon.