In the bowl of a stand mixer fitted with a dough hook, stir together Water (1 cup), Active Dry Yeast (2 teaspoon), and Granulated Sugar (1 teaspoon). Allow to ferment for 5-10 minutes.
Stir in 1 cup of the All-Purpose Flour (2 1/4 cup), Olive Oil (2 tablespoon), Freshly Ground Black Pepper (2 teaspoon), and Salt (1 teaspoon). Knead on low, adding remaining flour by the 1/4 cup, allowing each to be absorbed before adding more.
Knead for about 7 minutes, or until dough is smooth and pulls away from the side of the bowl.
Place dough in a large oiled bowl and cover with plastic wrap or kitchen towel. Allow to rise in a warm, draft-free area until double in size, approximately 1 hour.
Knead dough with your hands 2-3 times to deflate it, then divide into two rounds.
Line two large baking sheets with parchment paper and drizzle each with Olive Oil (2 tablespoon). Using half the dough for each baking sheet, flatten out dough onto parchment using your fingers. Press and stretch dough into an approximate 9 by 7-inch rectangle. If dough continues to shrink back after stretching, allow to rest for a few minutes and then continue to press out into desired shape.
Preheat oven to 450 degrees F (230 degrees C). Position two oven racks in the most central oven positions.
Rinse Leek (1) thoroughly, making sure to rinse any loose leaves. Trim ends of leek and cut into 1/8-inch slices.
In a medium-size skillet, heat Olive Oil (2 tablespoon) over medium heat. Add leeks and Salt (1/4 teaspoon) and sauté until leeks begin to soften, but do not brown, approximately 4-5 minutes. Remove from heat and set aside.
Sprinkle a quarter of the Shredded Mozzarella Cheese (1 1/2 cup) evenly over each dough round and top each with half of the sautéed leeks. Add another quarter of mozzarella cheese and half of the Parmesan Cheese (4 tablespoon) to each dough round. Season both rounds with additional salt.
Bake for 18-20 minutes, until dough is golden and cheese is starting to bubble. Swap trays on racks halfway through for uniform baking if baking both at the same time.
Remove flatbread from oven, cut into slices or wedges, and serve immediately. Enjoy!