Preheat your oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish with olive oil.

In a medium-sized bowl, combine Greek yogurt, minced garlic, lemon juice, chopped dill, olive oil, salt, and pepper, stirring until smooth.

In a small bowl, whisk cornstarch and chicken broth until dissolved, then incorporate into the yogurt mixture until slightly thickened.

Fold in drained cucumber, chopped red onion, cherry tomatoes, and crumbled feta into the tzatziki sauce until well-coated.

Add chicken pieces to the tzatziki mixture, ensuring each piece is well coated.

Transfer the chicken and tzatziki mixture into the prepared baking dish, spreading evenly.

Cover the dish with aluminum foil and bake for 25 minutes.

After 25 minutes, remove the foil, check the chicken for doneness, sprinkle with additional feta if desired and return to the oven uncovered for another 15-20 minutes.

Once baked, let the casserole rest for 5-10 minutes before serving.