Place flour in a large mixing bowl and make a well in the center. Add the water and stir in yeast. Allow it to dissolve, then stir it into the flour. Add the sunflower oil and soy sauce. Whisk the ingredients and start kneading the dough.
Once the dough comes together in a sticky blob, transfer it to a floured surface and knead it until you get an elastic, soft, but not sticky ball of dough. Add flour as needed.
Put it back into the bowl, cover with a kitchen towel and keep in a warm place until it almost doubles in size; at least 30 minutes.
Turn on your oven to 392°F (200°C) and grease a large baking sheet with oil. Set it aside.
Prepare the spinach
Heat about 1 tablespoon olive oil in a skillet over medium heat. Add the chopped spinach, sprinkle it with 1/2 teaspoon salt and cook, stirring occasionally, until wilted. This should take a few minutes tops.
Transfer the spinach to a sieve, placed over a bowl, to drain the excess liquid.
Add another tablespoon of olive oil to the skillet, once it warms up toss in the leeks and sprinkle with about 1 teaspoon of salt. Cover with a lid and let cook for a few minutes until they are tender and golden (to golden brown) in color. Set aside.
Assemble
When the dough has almost doubled in size, transfer it to a floured surface and roll it out in a square or rectangular shape that fits your pan. Transfer it to the baking sheet and lightly brush it with olive oil.
Scatter the drained spinach over the dough, add the sliced mozzarella, top with leeks and lastly sprinkle Parmesan over that.
Bake for 10-15 minutes, until the cheese is melted and bubbly. Once baked, let it sit for a few minutes, then serve cut in squares.
Notes
Aged mozzarella is the dry sister of classic mozzarella. It's usually sold tightly wrapped in plastic and shaped like a log. It isn't as moist as classic mozzarella and therefore won't add extra water to whatever dish you're cooking.