How to Make It
Preheat the oven to 350°. In a large nonstick skillet, heat the olive oil. Add the mushrooms and cook over high heat, stirring occasionally, until lightly browned, about 6 minutes. Add the shallot and cook over moderately low heat for 3 minutes. Add the spinach, thyme and nutmeg and cook over high heat until the spinach has wilted, about 2 minutes. Season with salt and pepper. Spread the spinach evenly in a 2-quart baking dish.
In a small saucepan, melt the butter. Whisk in the flour over moderately high heat. Add the stock and cream and whisk until thickened, about 5 minutes. Season lightly with salt and pepper and pour over the spinach. Arrange the polenta slices on top of the spinach in overlapping concentric circles, pressing to submerge the polenta slightly. Sprinkle the Gruyère on the polenta, cover with foil and bake for 40 minutes.
Preheat the broiler. Uncover the polenta and broil 6 inches from the heat for about 2 minutes, or until golden. Let stand for 10 minutes before serving.