16 scallions, trimmed
2 cups Fish Stock
1 375-ml bottle sauternes
4 4-oz. skinless center-cut sea bass filets,
4 large white mushrooms, trimmed,
cleaned, and sliced
8 tbsp. butter, cut into small pieces
50 muscat or other white grapes, peeled,
halved, and seeded
1. Trim green parts off scallions, leaving 2" of the pale green parts attached to the white bulb; set white parts aside. Slice one-third of the green parts crosswise (discard remaining green parts); set aside. Bring stock and 1⁄4 cup of the wine to a simmer in a large skillet over medium heat. Poach fish, partially covered, until halfway cooked through, about 1 1⁄2 minutes. Turn fish and continue poaching until just cooked through, about 1 1⁄2 minutes more. Divide fish between 4 warm shallow bowls, leaving poaching liquid in skillet, and cover fish to keep warm.
2. Bring poaching liquid to a boil over high heat, add remaining wine, and boil until thick enough to just coat the back of a wooden spoon, about 15 minutes. Add scallions and mushrooms, reduce heat to medium, and cook until scallion bulbs are soft, 8–10 minutes. Using a slotted spoon, divide scallion mixture between bowls with fish. Continue boiling sauce in skillet until syrupy, 1–2 minutes.
3. Remove skillet from heat and whisk in butter, 1 piece at a time, whisking thoroughly after each addition. Return skillet to heat and add grapes, a squeeze of lemon juice, and salt to taste. Heat sauce over medium-low heat until grapes are warmed through, about 1 minute. Pour sauce and grapes over fish and garnish with a sprig of chervil, if you like.