IMPROV: For a milder, creamier version, use a blend of Parmesan and Fontina instead of all Parm. (Semolina flour is sold at some supermarkets, Italian markets, and specialty foods stores.)
In large saucepan, bring to a boil milk, water, butter, and salt. Reduce heat to medium. Gradually whisk in semolina flour, then whisk until mixture is thick and smooth, about 5 minutes. Whisk in spinach, Parmesan cheese, eggs, nutmeg, salt, and pepper. Pour mixture into prepared baking dish; smooth top and sprinkle with 1/3 cup grated Parmesan cheese.
Bake gratin until puffed and golden, about 40 minutes. Cool 10 minutes before serving.
