In a large skillet over medium heat, cook the bacon to the desired crispness. Place the cooked bacon on paper towels to drain excess grease. Remove all but about 1 tablespoon of bacon grease from the skillet and lower the heat to medium-low.
In a medium-size mixing bowl, whisk together eggs, salt, black pepper, and green chiles.
Add egg mixture to the skillet. Scramble the eggs until they are no longer runny, stirring often. Add the bacon and spinach and cook for 1-2 minutes until the spinach has wilted.
Remove the egg, bacon, and spinach mixture and wipe the frying pan clean.
Add a little pad of butter or bacon grease to the pan and allow it to melt. Place one tortilla in the skillet (or cast iron griddle).
Cover the tortilla with ¼ cup of cheese and ¼ of the egg mixture, leaving an approximately ½ inch border around the tortilla. Top the egg mixture with another ¼ cup of cheese and cover with the second tortilla, cooked side down.
Cook for 60-90 seconds, letting the bottom layer of cheese melt and the tortilla get crispy.
With a spatula, lightly press down on the quesadilla before carefully flipping it over. Cook for an additional 60-90 seconds.
Remove from the frying pan and place on a cutting board. Allow quesadilla to cool for 1-2 minutes before cutting with a pizza cutter or sharp knife.
Repeat for the remaining quesadillas.
Serve quesadillas warm with fresh salsa, guacamole, or avocado slices.