Ironically, the key to a crispy quesadilla is less fat in the pan, not more. Too much fat will make your quesadilla soggy instead of crispy. Use just enough to coat the bottom of your skillet—about 1/2 tsp. of butter or oil. Warm it in the skillet over medium to medium-high heat.
Place one tortilla in the skillet and sprinkle all over with some of the cheese (depending on how many you are making).
Watch for the cheese to melt. Once the cheese starts to melt, lift a corner of the tortilla to check the underside. When the cheese has completely melted and you see golden-brown spots on the underside of the tortilla, the quesadilla is ready for the next step.
Add the sauteed vegetables on top and either fold over one tortilla and flip to cook both sides, OR add a full tortilla on top and proceed to finish.
Transfer to a cutting board with a spatula. If serving immediately, let cool for a minute or two for the cheese to set, then cut into wedges with a pizza cutter. If preparing several quesadillas for a crowd, slide the un-cut quesadillas onto a baking sheet and keep warm in a 200°F oven, then slice into wedges just before serving.
Wipe the pan clean if needed, melt another dab of butter, and continue cooking quesadillas as described above.
Add sour cream, cilantro/parsley on top ... include salsa and guacamole.