01 - In a large mixing bowl, combine the flour and salt. Pour in the lukewarm water and olive oil, stirring until a rough dough begins to form. Turn out onto a lightly floured surface and knead for 5 to 7 minutes until the dough is smooth and elastic. Cover with a damp cloth and let rest for 20 minutes.
02 - While the dough rests, heat 1 tablespoon of olive oil in a skillet over medium heat. Add the finely chopped onion and sauté until softened, about 2 to 3 minutes. Add the chopped spinach and cook until completely wilted. Remove from heat and let cool slightly, then stir in the crumbled feta, black pepper, and red pepper flakes. Mix thoroughly to combine.
03 - Divide the rested dough into 4 equal portions. On a lightly floured surface, roll each portion into a thin oval or rectangle approximately 1/8 inch thick.
04 - Spoon a quarter of the spinach and feta filling onto one half of each rolled dough piece, leaving a small border around the edges. Fold the uncovered half of the dough over the filling and press the edges firmly to seal.
05 - Heat a large skillet or griddle over medium heat. Lightly brush each gozleme with olive oil or melted butter on both sides. Cook for 2 to 3 minutes per side until golden brown and crisp.
06 - Transfer the cooked gozleme to a cutting board, slice into portions, and serve immediately while hot.