Heat half of the olive oil and sauté the chopped onions in it.
Add chopped spinach and let it steam, covered, until it collapses. Remove spinach from heat and let cool.
Meanwhile, whisk eggs with sour cream and mix into spinach along with remaining olive oil, garlic, pepper, some grated nutmeg and feta cheese. Season to taste with salt.
Now roll out the puff pastry into a rectangle on a floured work surface. Place the spinach filling in the center of the dough, brush the long sides of the dough with egg, fold and brush again with egg.
Prick with a fork several times. Bake in the preheated oven at 210 °C and finish the spinach strudel at 180 °C until golden brown.