How to Make It
Preheat the oven to 350°. Butter a deep 2-quart baking dish. Melt the butter in a large skillet. Add the squash and stir to coat. Add the water, cover and cook over moderate heat until the squash is tender, about 10 minutes. Uncover and cook over high heat until the water has evaporated, about 3 minutes. Transfer the squash to a large bowl, mash it and let cool to room temperature.
Add the garlic and crème fraîche to the squash and season with salt and pepper, then whisk in the egg yolks. Spread the squash in the prepared baking dish and scatter the cheese over the top.
Cover the squash with foil and bake for about 40 minutes, or until almost set. Increase the oven temperature to 425°. Uncover the squash and bake in the upper third of the oven for about 12 minutes, or until the cheese is browned. Remove the gratin from the oven and let stand for 10 minutes before serving.