In a large skillet, heat olive oil over medium heat. Add the chopped spinach and sauté for about 2-3 minutes until wilted.
In a bowl, whisk together the eggs, salt, pepper, and red pepper flakes, if using.
Pour the egg mixture into the skillet with the spinach. Stir gently and cook until the eggs are just set but still slightly soft, about 3-4 minutes. Remove from heat.
On one half of each tortilla, sprinkle some mozzarella cheese, followed by a portion of the spinach-egg mixture, and then crumbled feta cheese on top.
Fold each tortilla over to create a semi-circle. Wipe the skillet clean and return it to low heat. Cook the quesadillas in the skillet for about 3-4 minutes on each side or until golden brown and the cheese has melted.
Remove the quesadillas from the skillet and slice them into wedges. Garnish with fresh herbs if desired.