Add the sliced red onion and sauté for about 3 minutes until it softens and begins to turn translucent.

Add the chopped spinach to the skillet, sprinkle with salt, pepper, garlic powder, and oregano, and cook until the spinach wilts and aromas rise, about 2–3 minutes.

Remove the sautéed spinach and onion mixture to a bowl, then stir in lemon juice to brighten the flavor.

Wipe the skillet clean with a paper towel and return it to medium heat.

Lay one tortilla flat in the skillet and sprinkle a generous layer of shredded mozzarella across one half of the tortilla.

Add half of the sautéed spinach mixture evenly on top of the mozzarella layer.

Scatter the crumbled feta over the spinach, and add olives if using them.

Sprinkle a bit more mozzarella on top to help bind the filling.

Fold the tortilla over so it forms a semicircle enclosing the filling.

Cook the quesadilla for 2–3 minutes on the first side until the bottom is golden brown and crisp.

Carefully flip the quesadilla with a spatula and cook the other side until it matches in color and the cheeses inside are melted.

Transfer the finished quesadilla to a cutting board and let it rest for a minute before slicing.

Repeat the process with remaining tortillas and filling ingredients.

Cut each quesadilla into wedges using a sharp knife or pizza cutter and serve while hot.