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.