Prepare the spinach dip by placing the spinach and artichokes in a large sauté pan sprayed with cooking spray. Cook over medium heat until spinach is wilted. Add garlic and sauté an additional 1-2 minutes. Turn off heat and add yogurt and cheeses. Stir to combine. Spread half of the spinach artichoke dip over two tortillas. Top with 1/2 cup shredded chicken if using.
Top each quesadilla with the additional tortilla, then place it in the same skillet used to cook spinach artichoke dip, (wipe pan with paper towel if needed). Cook the quesadilla over medium heat until brown and crispy on one side, then flip and cook and until brown and crispy on the second side.
Slice the quesadilla into two or three pieces with a pizza slicer or large knife, and serve.