Heat a splash of olive oil in a skillet over medium heat. Sauté the red onion for about 2 minutes until softened, then add the spinach and cook until it wilts, about 1-2 minutes more.

Take a tortilla and lay half of it flat on a plate. On that half, layer the sautéed spinach, crumbled feta, and shredded mozzarella.

Carefully fold the tortilla in half. In a preheated skillet, add a little oil, and cook the quesadilla for about 3-4 minutes on each side, until golden brown and the cheese is melted.

Remove the quesadilla from the skillet and let it rest for a minute. Cut it into wedges and serve with optional garnishes like tzatziki or fresh parsley.