Heat 1 tablespoon olive oil in a skillet over medium heat. Add the thinly sliced red onion with a pinch of salt and cook, stirring occasionally, until the onion softens and begins to caramelize — about 6–8 minutes.

Stir in the minced garlic and cook for 30 seconds until fragrant. Add the chopped spinach and cook until just wilted, then toss in the dried oregano, lemon zest if using, and season lightly with salt and pepper. Remove from heat.

Lay a tortilla flat. Sprinkle a thin layer of shredded mozzarella over half of the tortilla, then spoon on a generous layer of the spinach and onion mixture. Crumble feta on top and add any optional extras like kalamata olives or sun-dried tomatoes. Fold the tortilla over to create a half-moon.

Heat a nonstick skillet over medium-low heat and add a small pat of butter or a drizzle of olive oil. Place the folded quesadilla in the pan. Cook 3–4 minutes per side, pressing gently with a spatula, until the tortilla is golden and crisp and the cheeses are melted.

Transfer to a cutting board and let rest for a minute, then slice into wedges. Repeat with remaining tortillas.

Serve immediately with yogurt-based tzatziki, lemon wedges, or a simple tomato salad.