Meanwhile, in a large, deep skillet, melt the butter in the oil. Add the mushrooms, season with salt and pepper and cook over high heat, stirring occasionally, until tender, 20 minutes. Add the onion and garlic and cook, stirring, until the mushrooms are deeply browned, 8 minutes. Add the wine and cook until evaporated. Stir in the stock, tarragon and thyme and bring to a boil. Season with salt and pepper.