While the dough is relaxing, heat olive oil in a frying pan. Add onions and sauté on a low flame for 8-10 minutes, stirring regularly. When the onions are very soft and starting to brown and caramelise, add in the apple cider vinegar. Cook until the liquid evaporates, another 3-5 minutes. Season with salt and remove the pan from the heat.
Preheat the oven to 200C and line a baking tray with baking paper
On a very well-floured surface, roll out the dough into a 25cm disk and transfer the dough disk to the lined baking tray.
On a very well-floured surface, roll out the dough into a 25cm disk and transfer the dough disk to the lined baking tray.
Bake for 20 minutes, rotating the pan halfway through or until the crust is brown.
Remove from the oven, slice and sprinkle some chopped rosemary (if using) over the pizza. You can also add more feta if desired. Serve immediately.