In a large skillet, heat 1 tablespoon of olive oil over medium heat. Add the finely diced onion and sauté until it becomes translucent, about 3-4 minutes, stirring occasionally.
Incorporate the minced garlic into the skillet, cooking for an additional minute until it becomes fragrant, ensuring not to let it burn.
Add the chopped spinach to the pan and cook, stirring frequently, until it wilts down, about 2-3 minutes. Season the mixture with salt, black pepper, and red pepper flakes if you prefer a bit of heat. Once cooked, remove the skillet from heat.
In a mixing bowl, combine the wilted spinach, sautéed onion, and garlic with the crumbled feta and shredded mozzarella cheese. Mix thoroughly to ensure even distribution of ingredients.
Heat a clean skillet over medium heat. Brush one side of a tortilla with olive oil, then place it into the skillet, oiled side down.
On one half of the tortilla, generously spread a portion of the spinach and cheese filling, making sure to cover it well.
Carefully fold the tortilla over to form a half-moon shape, pressing down lightly to seal the edges. Cook for about 2-3 minutes until the bottom is golden brown and the cheese has begun to melt. Gently flip the quesadilla and cook for an additional 2-3 minutes on the other side until it reaches a similar golden brown color.
Remove the quesadilla from the skillet and place it on a plate. Repeat the process with the remaining tortillas and filling until all quesadillas are cooked.
Allow the quesadillas to cool for a minute before slicing each one into wedges for easier serving.
Notes
Serve with yogurt or tzatziki sauce and garnish with fresh herbs.