How to Make It
Preheat the oven to 425°. Arrange the squash rounds on 2 rimmed baking sheets and brush with the olive oil. Season with salt and pepper and bake for 20 minutes, or until barely tender. Increase the oven temperature to 475°.
Meanwhile, set a medium saucepan over moderately high heat. Add the spinach, 1 handful at a time, and cook, stirring frequently, until wilted. Drain the spinach, squeeze dry and coarsely chop.
In the same saucepan, bring the heavy cream to a boil, then simmer over low heat until slightly thickened, about 3 minutes. Add the spinach and simmer, stirring occasionally, until creamy, but not runny. Season with salt and pepper, transfer to a plate and let cool.
Lightly oil a large, shallow baking dish. In a small bowl, stir the crème fraèche into the Parmesan. Spread the Parmesan cream on 8 squash rounds. Arrange 8 of the remaining rounds in the baking dish. Spoon a scant tablespoon of the creamed spinach over each round. Repeat this layering 2 more times and top with the rounds spread with Parmesan cream.
Cover the gratins with foil and bake until heated through, about 10 minutes. Preheat the broiler. Uncover and broil the gratins for 2 minutes, or until browned. Transfer to plates and serve.