Warm the milk in a pot on the stove to between 100 and 105 degrees F. Pour in a small bowl, and mix in sugar and yeast. Cover and leave on counter for 10 minutes to activate yeast.
Mix flour, salt, and 1 ½ tbsp olive oil in a large mixing bowl. Add yeast mixture and knead the dough for 5 to 10 minutes. Add more flour or water as needed. Once a stretchy dough is made, cover and leave on counter for an hour.
Add your onions to a large bowl of very cold salt water. Let them soak for 30 minutes, then drain the water and dry the onions.
Separate the dough into 2 balls. Use 2 tbsp of olive oil to grease a large skillet (cast iron or stainless steel with a metal handle). Press 1 ball into the skillet, stretching into a disc that covers the bottom and an inch up the side of the skillet.
Layer the mozzarella evenly over this layer of dough, and sprinkle the feta cheese evenly.
Roll out the second ball of dough, lay it on top of the skillet. Push the edges down and seal the two layers of dough together, encasing the cheeses in the middle.
Brush the top with a little olive oil, about 1 tbsp. Sprinkle parmesan cheese and garlic over the top. Cover the top with the thin slices of onion. Sprinkle fresh rosemary leaves and dried oregano over the top.
Bake in the oven at 450 degrees F for 20 to 25 minutes, or until golden brown on top with crispy onions. Enjoy!
Make sure your skillet is oven safe before putting it in the oven.