I make quesadillas at work all the time. I also have to make about 300 a clip
I use a 10 inch flour tortilla. I have the option of using a flattop grill or a flame grill. I find my flame grill slats just a bit too far apart for my needs, but can use it if I have to. For a home grill, you can always use one of those grilling pans that have the little holes, giving you the fire taste that is supreme, but giving you the security of not falling in.
Place the tortilla on your work surface. Place about 1/3 c shredded cheese on one half of the shell. At this point, add whatever it is you like. Generally, I'll add sauteed onions, diced, sauteed peppers (green, red, jalepeno), S&P, a spritz of lime juice and some chopped cilantro. Fold it over and give it a little press.
Place it onto the hot grill. I don't add any fat to the tortilla itself, but I do oil the grill. Then, it's just like making a grilled cheese sandwich. Flip it after a moment or two, when it begins to turn a golden brown. Cook for another minute, or till that side is also browned.
Remove to platter and hold for about a minute before slicing into 4 triangles. Serve with your favourites - sour cream, salsa, guacamole.
If you want to make several, keep the cooked ones warm and well wrapped till you cut. Don't cut till you are going to serve.