Instructions

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.