Preheat oven to 350°F. Butter a shallow 1-quart baking dish.
In a large bowl, combine spinach, half-and-half, eggs, 1 cup Gruyere, 2 teaspoons salt, ⅛ teaspoon pepper, and nutmeg; stir to combine. Spread evenly in prepared baking dish, and top with remaining ½ cup Gruyere.
Bake until set and top is golden brown, 30-35 minutes.
To make it in advance: Bake up to 1 day ahead and refrigerate. Bring to room temperature, then cover with foil, and bake at 350°F for 20 minutes. Uncover and bake 20 minutes more.
Recipe by Life at Cloverhill at https://lifeatcloverhill.com/2010/11/spinach-and-cheese-puff.html