In a medium bowl, combine Greek yogurt, olive oil, lemon juice, minced garlic, and Greek spices. Whisk until smooth, then add the chicken breasts, ensuring they're well-coated. Cover and refrigerate for at least 30 minutes, or up to overnight.

Preheat your grill or skillet over medium-high heat. Remove the marinated chicken from the refrigerator and let it come to room temperature for about 10 minutes. Grill or pan-sear the chicken for 6-7 minutes on each side until it reaches an internal temperature of 165°F. Let it rest before slicing.

While the chicken is cooking, prepare your base. If using rice or quinoa, cook according to the package instructions, usually simmering for 15-20 minutes. For salad greens, wash and chop your choice of mixed greens.

Chop your fresh vegetables. Dice cucumbers, slice tomatoes, and thinly slice red onions. Arrange them on a cutting board, ensuring they maintain their crispness.

Start assembling the bowls with a scoop of your rice or quinoa base at the bottom. Layer the sliced grilled chicken on top, followed by your chopped cucumbers, tomatoes, and red onion.

Drizzle tzatziki sauce over the assembled bowls. You can also serve it on the side.

For added flavor, include optional garnishes like feta cheese, olives, or fresh herbs. Serve your Greek Chicken Gyro Bowls immediately.