Combine the spinach, spring onions, spearmint and dill, all finely chopped, in a bowl. Sprinkle with salt and work the greens by hand pressing water out, until they shrivel. Drain the water off and place in a clean bowl.
Season with pepper, add the olive oil, sour mizithra cheese and finely chopped onion to the mixture. Mix all the ingredients by hand to obtain a uniform mixture. Cover, allow to stand in the refrigerator, until you have prepared the dough. In a clean bowl place the flour, make a hole in the middle and fill with olive oil, salt and lemon juice. Gradually add some tepid water and continue working by hand into a somewhat soft dough. Allow to stand for 10 minutes. Then divide it into pieces and roll them out into fine pastry sheets. Cut small circles on each sheet and place a teaspoon of filling in the centre of each one. Shape into crescents and press the edges with the tines of a fork so as to seal the small pastry envelopes. Heat olive oil and transfer the onion pies one by one into it. Fry them until slightly brown. Remove with a perforated skimmer and place them on a paper towel to drain off the oil.
Serve hot or cold.