Preheat the oven to 350°F and line a baking sheet with parchment paper.
In a large bowl, add the ground turkey, egg, bread crumbs, red onion, garlic, mint, oregano, salt, and black pepper. Using your hands or a fork, gently mix until everything is well incorporated.
Roll the meatball mixture into 2-tablespoon balls, placing them onto the baking sheet once formed, until all the turkey is used, and you have about 16 mini meatballs.
Transfer the baking sheet to the oven to bake for 14-16 minutes until the turkey meatballs are cooked through to an internal temperature of 165°F when tested with a meat thermometer.
While the turkey meatballs are cooking, cook the orzo. Bring a medium pot of well-salted water to a boil, add the orzo, and cook as per the package directions until tender, and then drain. Return to the pot, drizzle with 2 tablespoons of olive oil, and toss until well coated.
(Note: Skip this step if you are using store-bought tzatziki.) While the orzo is cooking, make the tzatziki sauce. Cut ¾ of the cucumber lengthwise, use a small spoon to remove the seeds, then use a box grater to shred the cucumber into fine pieces. Place the shredded cucumber into a paper towel and, using your hands while holding it over the sink, squeeze the paper towel to remove as much excess water from the cucumber as possible. Then transfer the cucumber to a small bowl with the Greek yogurt, minced garlic, lemon juice, dried dill, dried parsley, and salt, and stir to combine. Taste and adjust seasoning with salt and pepper as needed, then set aside.
In a medium bowl, add the halved cherry tomatoes, remaining ¼ diced cucumber, diced red onion, kalamata olives, and feta cheese, season with salt and pepper, and toss until well combined.
Once the turkey meatballs are cooked, prepare the bowls. Layer each bowl with orzo and veggies, top with cooked meatballs, a dollop of tzatziki sauce, a sprinkle of feta cheese, and fresh mint leaves.
Any leftover turkey meatballs can be stored in an airtight container in the fridge for up to 4 days or in the freezer for up to 3 months. Leftover orzo, veggies, and tzatziki sauce can be stored in separate airtight containers in the fridge for up to 5 days.