To make the tzatziki sauce, combine Greek yogurt, cucumber, garlic, dill, lemon juice, lemon zest and mint in a small bowl; season with salt and pepper, to taste. Drizzle with olive oil. Refrigerate for at least 10 minutes, allowing the flavors to meld; set aside.
Preheat oven to 400 degrees F. Line a baking sheet with parchment paper.
Top tortilla with spinach, sun dried tomatoes, olives and cheeses, and then top with another tortilla. Repeat with remaining tortillas to make 4 quesadillas.
Place quesadillas onto the prepared baking sheet. Place into oven and bake until the cheese has melted, about 8-10 minutes.
Serve immediately with tzatziki sauce, garnished with dill, if desired.