In a skillet, heat 1 tablespoon of olive oil over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes.
Add the minced garlic and sauté for an additional minute until fragrant.
Stir in the chopped spinach and cook until wilted, about 2-3 minutes. Season with salt, black pepper, and red pepper flakes (if using). Remove from heat.
In a bowl, combine the wilted spinach mixture with crumbled feta and shredded mozzarella, mixing well.
Heat a clean skillet over medium heat. Brush one side of a tortilla with olive oil, then place it in the skillet, oiled-side down.
Spread a generous amount of the spinach and cheese mixture on one half of the tortilla.
Fold the tortilla over to create a half-moon shape. Cook for about 2-3 minutes until golden brown and the cheese is melted, then carefully flip and cook for another 2-3 minutes on the other side.
Remove from the skillet and repeat with the remaining tortillas and filling.
Let the quesadillas cool for a minute before cutting into wedges.
Notes
Serve warm with yogurt or tzatziki sauce and garnish with fresh herbs.