Preheat oven to 400 degrees. In a large ovenproof skillet, heat olive oil and sauce shallots and garlic until soft (but not brown). Add mushrooms and sauce until they begin to soften.
Arrange the haddock on the mushrooms and shallots in a single layer. Season with salt and pepper, add wine, and place in oven, and bake about 8 minutes (or until the fish just turns opaque in the middle).
Transfer fish and mushrooms to a warm serving platter and cover to keep warm.
add vinegar to the skillet and cook rapidly over high heat until the liquid just coats the bottom. Swirl in the butter with a whisk and reduce the temperature, cooking gently until the sauce begins to thicken.
Stir in the parsley, season with salt and pepper, pour over the fish and serve.