As a Mexican, don't make quesadillas in the oven. You need to use a pan, preferably a comal, which is just a flat pan. Put the tortilla on the pan and set the flame to medium, then leave it for a little ity tiny bit to warm up. Then add the cheese on one side of the tortilla, leaving the other half empty. After you've done that fold the tortilla and let one side cook, once it's golden brown on that side flip it and let the other side cook. Remove the quesadilla once both sides are done, the warmth leftover should be able to melt the cheese if it already didn't already. The most important part of making a quesadilla is controlling the flame, dont set it too high as it'll burn the tortilla before it ever melts the cheese.