Defrost the spinach. Preheat the oven to 200°C. Place a rectangular ovenproof dish in the oven and preheat for 5 minutes.
Beat the egg in a bowl until very foamy. Mix the flour and baking powder, add to the egg alternately with the milk and mix until smooth. Meanwhile, mix the spinach and Parmesan into the batter, season with pepper and nutmeg.
Add the butter to the hot dish, swirl to distribute it evenly. Pour in the batter and smooth it out. Bake in the oven on the middle rack for 15 minutes.
Let the pastry cool in the pan for 30 minutes.
Then turn out onto a board. Divide into triangles. Delicious as finger food or as a chowder in a soup, good also as a side dish with fish and salad.