Best fresh. Cooked quesadillas keep 2 days and reheat well in a dry skillet or air fryer; the microwave makes them limp. You can assemble them uncooked and refrigerate for a day.
Use less filling than you think, cheese on both sides, and let it rest a minute before cutting so the cheese sets.
Oil gives a cleaner crisp and won't burn. Butter tastes better but browns fast — use it over lower heat.
Yes, 425°F for about 10 minutes, flipping halfway. Good for making several at once, though slightly less crisp than a pan.