Heat a large, nonstick pan on medium. Lay out two flour tortillas and sprinkle cheese shreds over half. Sprinkle ingredients of your choice, and one more layer of cheese. Fold tortillas in half, spray one side lightly with cooking spray, and place those sides down in the pan. Two of them fit perfectly in a round pan. Place a heatproof plate on top and weigh it down with two heavy cans, one over each quesadilla.
Check one to see if it's lightly browned, and when it is, spray the uncooked sides with cooking spray, and flip the quesadillas. A fish turner helps. Replace the plate and cans and cook for a few more minutes.
When they're done, use a pizza cutter to cut them. I recommend cutting each one in half and then making one more cut in the same direction in each one so all pieces are folded over and will stay together. Serve with sour cream and salsa.