Heat half the olive oil in a skillet over medium heat. Add diced red onion and sauté for 2 minutes.

Add minced garlic and chopped spinach to the pan. Cook until spinach is wilted and liquid has evaporated. Season with oregano, salt, and pepper. Remove from heat.

Lay tortillas flat. On one half of each tortilla, layer mozzarella, the spinach mixture, and crumbled feta. Top with a little more mozzarella.

Fold the tortillas over to close. Wipe the skillet clean and add the remaining oil or a pat of butter.

Place quesadillas in the pan. Cook for 3-4 minutes per side until golden brown and cheese is fully melted.

Remove from pan, let rest for 1 minute, slice into wedges and serve immediately.