Clean and chop the spinach and onions and put them to stew with a little butter. If the finished mixture has a slightly bitter taste, add 1 tsp sugar.
Remove the mixture from the heat and add the crumbled feta cheese, eggs, salt if necessary, and mix well. Smear an oven dish with oil thoroughly and with flour generously so that after baking, you get a thick crust.
Place 2-3 sheets of phyllo pastry on top. Pour the spinach mixture on top and put another 2-3 sheets of phyllo pastry. Sprinkle it with oil and a lot of flour once again, to get a crust on top.
Bake the zelnik in a moderate oven until it gets a nice tan and serve it, cut into square pieces.