Thaw the frozen spinach and then place in a colander. Press the spinach against the sides of the colander to squeeze out as much moisture as possible. Place the drained spinach in a large bowl.
Cut the red onion into very thin slices. Cut the roasted red pepper into small strips, roughly chop the kalamata olives, and roughly chop the parsley. Place all four ingredients in the bowl with the spinach.
To the bowl also add the oregano, crumbled feta, and shredded mozzarella. Stir very well until everything is evenly distributed.
Divide the filling among all ten tortillas, spooning it into one half and then folding the bare half over top (I used roughly 3/4 cup per tortilla). Heat a non-stick skillet over medium to medium/low heat. Cook the quesadillas on each side until the outside is golden brown and crispy and the cheese inside has melted.
These quesadillas can also be frozen prior to cooking and then warmed in a skillet straight from the freezer. When cooking the quesadillas from the frozen state, use a lower heat to allow the inside time to thaw and melt before the outside becomes crispy.