Heat olive oil in a large skillet over medium heat. Add the sliced red onion and sauté for about 3-4 minutes until softened.
Add the chopped spinach to the skillet and cook for another 2 minutes until wilted. Stir in garlic powder, dried oregano, salt, and pepper. Remove from heat.
Lay one tortilla flat on a clean surface. Spread half of the spinach and onion mixture evenly over the tortilla. Sprinkle half of the feta and mozzarella cheese on top.
Place the second tortilla on top and press down gently.
Wipe the skillet clean and return it to medium heat. Carefully transfer the quesadilla to the skillet. Cook for about 3-4 minutes on one side until golden brown and the cheese starts to melt.
Flip the quesadilla carefully and cook for another 3-4 minutes on the other side until golden brown and the cheese is fully melted.
Remove from the skillet and let it cool for a minute before slicing into wedges. Serve with sour cream or tzatziki if desired.