Heat 1 tablespoon of olive oil in a skillet over medium heat. Add chopped spinach and sliced red onion. Sauté for 3-4 minutes until spinach wilts and onion is translucent. Season with salt, pepper, and oregano. Remove from heat and let cool slightly.

In a mixing bowl, combine the sautéed spinach mixture with crumbled feta and shredded mozzarella. Stir until well mixed. Take a tortilla, spread a quarter of the mixture over half, and fold it in half. Repeat with remaining tortillas.

Wipe the skillet clean and return to medium heat. Add remaining tablespoon of olive oil. Cook each folded quesadilla for 2-3 minutes on each side until golden brown and cheese melts. Cut into wedges and serve.