Clean and chop the spinach finely, crumble the feta. Put the olive oil and butter in a casserole and heat it on the stove. In a bowl, beat the eggs, add the milk, yoghurt, flour and baking powder. Stir until homogeneous.
Smear a 14 1/4″ (36 cm) diameter tray with a little of the grease. Sprinkle a phyllo pastry sheet with a little of the grease, put milky filling on it and distribute, place a 2nd sheet on top, smear it with filling and sprinkle spinach and feta again.
Wrap the sheets into a roll and place in the middle of the tray in a spiral.
Repeat this process for the rest of the sheets, smear 1 sheet with grease and a little filling, place a 2nd sheet, smear with filling and sprinkle spinach and feta. Roll up and place in the tray, wrapping around the middle one.
Distribute leftover filling over the arranged sheets. Put in the oven and bake at 338°F (170 °C) until reddened for about 45 min. Serve with a spoon of yoghurt.