Preheat the oven to 400-degrees.
Prepare baking sheets with non-stick cooking spray or line them with parchment paper or silicone baking mats.
Get out one tortilla shells per quesadilla. Cover on side of it with the margarine or butter. (For spray margarine, I use 8 to 10 squirts and smear it around with my clean hands.)
Please the shells buttered side down onto your prepared baking sheet(s).
Put your filling ingredients onto the top of the shells. Top them with cheese. I use 3/4 Mexican blend or colby jack cheese and 1/4 Italian blend or mozzarella cheese. The amount of cheese varies based on the size of your shell and your family's preferences.
Then add whatever ingredients your family prefers. I love chicken bacon ranch using precooked, refrigerated chicken, precooked bacon cooked until crispy in the microwave, shredded cheese, ranch dressing, tomato, onion and roughly chopped spinach.
Place another tortilla on top. Lightly cover it with margarine or butter, just as you did the bottom shells.
Place into your preheated oven for 6 minutes.
Pull out the quesadillas and carefully flip them over.
Bake for 6 more minutes or until the edges are a nice golden brown.
Use a pizza cutter or sharp knife to slice and serve hot. We usually serve ours with sour cream because we love it!