Dissolve the yeast and sugar in 1 cup of warm water (40-45 degrees c). Allow to sit undisturbed for 10 minutes. Add the flour to the bowl of a standing mixer fitted with a dough hook. Add the yeast mixture, salt, and olive oil. Start the mixer on low. Once the dough has come together, continue to knead for 5-6 minutes on medium speed. The dough should be elastic and slightly tacky.
Remove the dough from the mixer. Form into a ball and add to a large, lightly oiled bowl. Cover with a tea towel and let rise in a warm place for 1.5 to 2 hours, until the dough has doubled in size.
To make the filling, bring a large pot of water to a boil. Blanch the spinach for 30 seconds, then drain and rinse under cool water. Squeeze as much water out of the spinach as possible (if you have a potato rice - it also doubles as a great way to squeeze out the water)*. Once you think you've squeezed enough, squeeze some more.
Finally, between several layers of paper towels, squeeze the last little bit of of excess liquid that you can. Transfer to a cutting board and roughly chop. In a medium-sized bowl, add the chopped spinach, onion, and feta, 1 tablespoon of olive oil, lemon zest, and lemon juice.
Add the salt and pepper just before you're ready to use the filling (mixing the salt with the onions at the last minute prevents the onions from releasing additional moisture into the mixture). Once the dough has risen, divide it in half. On a lightly floured surface, roll out half of the dough to ⅛-inch thickness. Using a 3-inch round cookie cutter or large glass, cut out the rounds (don't discard the scraps).
Preheat the oven to 220 degrees c. Line a baking sheet with foil and lightly brush with olive oil. (Remember to add the salt and pepper to the filling, if you haven't done so already). Working with one round at a time, gently stretch the dough to give you a little more working surface, much like you would if you were stretching dough for a pizza. Place 1 heaping tablespoon of filling in the center - be sure to keep the edges free of any stray pieces of filling.
Form a triangle by raising three sides of the dough up and over the filling. Pinch each seam, securing the filling. Place each completed triangle on the prepared baking sheet. Lightly brush the triangles and leftover dough scraps with the remaining olive oil and sprinkle the tops with sesame seeds. Bake for 16 -18 minutes until golden brown. Serve warm or at room temperature with yoghurt dip on the side. Enjoy!