Combine flour, baking powder, and salt in a bowl. Add oil, egg, and yogurt. Add milk gradually until a smooth, pliable dough forms that does not stick. Form into a ball, cover in plastic wrap, and place dough in the refrigerator to rest for about 1 hour.
Preheat the oven to 350 degrees F (175 degrees C). Oil a glass baking dish.
Bring a large pot of water to a boil. Add spinach and cook for 1 minute. Drain and let cool.
Heat oil in a large skillet over medium heat. Add onion and cook until transparent, 3 to 5 minutes. Remove from heat and let cool.
Chop cooled spinach and combine with onion in a bowl. Add feta cheese, Cheddar cheese, then halloumi cheese. Season with salt and pepper. Add eggs.
Take dough from refrigerator and knead on a clean surface. Divide into 2 pieces, 1 slightly larger than the other. Roll out larger piece of dough to cover bottom and sides of the prepared baking dish, using your hands to stretch the dough.
Spread filling into dough-lined baking dish. Roll out remaining dough to fit and place on top of filling. Pinch sides to enclose filling. Cut a cross on top and brush lightly with milk.
Bake in the preheated oven until golden on top and risen, about 45 minutes.
