Prepare the Dough:

• In a large bowl, combine the flour and salt.
• Add the melted butter and mix until evenly distributed.
• Gradually add the warm water while stirring, until a soft dough forms. Adjust water if needed.
• Knead the dough on a floured surface for 10 minutes, until smooth and elastic.
• Cover the dough with a damp cloth and let it rest for at least 30 minutes.

Prepare the Filling:

• Heat olive oil in a skillet over medium heat. Add the chopped onion and garlic, sautéing until translucent.
• Add the spinach to the skillet and cook until wilted and excess water evaporates.
• Season with salt, pepper, and red pepper flakes (if using).
• Remove from heat and allow to cool slightly. Stir in the crumbled feta cheese.

Assemble the Flatbreads:

• Divide the dough into equal portions and roll each into a ball.
• On a floured surface, roll out each dough ball into a circle about 1/4 inch thick.
• Spoon the spinach and cheese filling onto one half of the dough. Fold the other half over the filling to form a semi-circle. Seal the edges by pressing or crimping with a fork.

Cook the Flatbreads:

• Preheat a skillet or griddle over medium heat and lightly grease it with oil or butter.
• Place the flatbreads in the skillet and cook for 3-4 minutes on each side, or until golden brown and cooked through. Adjust heat as necessary to prevent burning.