Make the sauce: In a medium bowl, stir together Greek yogurt, cucumber, dill, and lemon juice. Season to taste with salt, pepper and red pepper flakes.
In a large skillet over medium heat, heat olive oil. Add one pita half, cut-side up. Top with a handful each of mozzarella and chicken. Sprinkle with feta, red peppers, olives and dill. Top with the matching pita half, cut-side down.
Cook until the cheese is melted and the bottom pita is golden, about 2 minutes. Carefully flip and continue to cook until the second side is golden, about 2 minutes more.
Repeat with remaining ingredients. Cut quesadillas into wedges and serve warm with Tzatziki sauce.
Recipe by 5* trending recipes with videos at https://food.theffeed.com/2019/01/06/greek-quesadillas/