This recipe was provided by Tom Colicchio, a restaurant owner and head judge of the popular Bravo series "Top Chef."
Wrap bay leaf, rosemary, 2 sprigs thyme, peppercorns, and fennel seeds in a piece of cheesecloth and tie to form a bouquet garni; set aside. In a large saucepan, heat 1 tablespoon oil over medium-high heat. Add onion, fennel, celery, and garlic; saute until tender, 6 to 8 minutes. Pour over wine and stock, add bouquet garni, and bring to a boil. Immediately reduce heat to a simmer and cook until the liquid thickens to a sauce consistency, 45 to 60 minutes. Strain through a fine sieve, reserving liquid.
Place cipollini onions in a roasting pan over medium heat. Drizzle with 1/2 cup oil, and season with salt and pepper. Toss to combine. Cook for 5 minutes, add mushrooms, and cook until vegetables are tender; set aside.
Preheat oven to 375 degrees. In a large skillet, heat remaining 2 tablespoons oil over high heat. Pat fish dry with paper towels and season both sides with salt and pepper. Place in skillet and cook until browned on both sides.
Add reserved liquid to a Dutch oven. Whisk in butter, tarragon, remaining 2 sprigs thyme, and vinegar. Add mushroom mixture and fish. Cover, and transfer to oven. Cook until just heated through, 10 to 12 minutes.
Transfer fish to shallow bowls. Spoon sauce over fish, and serve.