Heat a skillet over medium heat and add a splash of olive oil. Once shimmering, add the chopped spinach and cook, stirring, until wilted and tender, about 2-3 minutes. Remove from heat and set aside.
Lay a tortilla flat on a clean surface and evenly sprinkle half of the crumbled feta over one half of the tortilla.
Spread a portion of the cooked spinach over the feta, covering the same half of the tortilla.
Fold the tortilla in half, pressing gently to encourage fillings to stay inside.
Place the folded tortilla on the heated skillet and cook for about 3-4 minutes, or until golden brown on the bottom. Flip carefully with a spatula and cook the other side until also golden and crispy, about 3 more minutes.
Remove the quesadilla from the skillet and let it rest for a minute before slicing into wedges. Repeat with the remaining tortillas and fillings.
Serve the quesadillas warm, with a crispy exterior and a warm, cheesy, and green-filled interior. Enjoy immediately for the best texture and flavor.