1 1/2 pounds skinless halibut fillets, cut into 2-inch pieces
2 tablespoons all-purpose flour
In a large saucepan, melt 2 tablespoons of the butter. Add the chopped shallot and garlic and cook over moderate heat until softened, about 3 minutes. Add the squash, parsnip and turnip and cook over moderately high heat until they begin to soften, about 5 minutes. Season with salt and pepper. Add the wine, thyme sprigs and bay leaf and cook until the wine reduces by half, about 2 minutes. Add the stock, cover and simmer until the vegetables are tender, about 10 minutes.
Meanwhile, in a large nonstick skillet, heat the olive oil. Season the halibut pieces with salt and pepper and dust them lightly with the flour. Add the halibut to the hot skillet and cook over moderately high heat until light golden, about 2 minutes per side.
Add the fish to the vegetables and simmer for 2 minutes. Using a slotted spoon, transfer the fish and vegetables to shallow bowls. Discard the thyme sprigs and bay leaf. Swirl the remaining 1 tablespoon of butter into the broth. Season with salt and pepper and spoon over the stew. Garnish with the thyme leaves and serve.