Chop the onions finely and put them to saute in a little oil. Add the spinach. Braise.
Add the finely chopped spearmint, a little salt, braise lightly and leave it to cool.
Beat the eggs with the feta cheese, you need to obtain a thick mixture. Cut the phyllo pastry sheets in half.
Take 2 sheets at a time and smear them with oil, put 2-3 piles of spinach, while placing egg mixture in between the piles. Wrap into a roll.
Arrange the rolls in a baking form, put pieces of butter on top and bake in a preheated 356°F (180 °C) oven until golden.