In a large bowl, whisk together greek yogurt, olive oil, lemon juice, minced garlic, oregano, cumin, salt, black pepper, and optional red pepper flakes
Toss in the chicken thighs, making sure they’re well coated. Cover and refrigerate for at least 1 hour.
Preheat your oven to 375°F (190°C).
Cut a yellow onion in half and place one half, cut side down, in the center of a foil-lined baking sheet or oven-safe dish.
Insert the bamboo skewer into the onion, standing straight up.
Stack the marinated chicken thighs one on top of the other, pressing them down slightly to create a tower.
Once all the chicken is stacked, place the second half of the onion on top to help secure everything in place (optional) - I have tried with and without.
Transfer the baking sheet to the oven and roast for 75 minutes, or until the chicken reaches an internal temperature of 165°F (75°C). Depending on your oven, could take up to 90 minutes.
About 30 minutes in, baste the chicken with the juices that have pooled at the bottom for extra moisture and flavor.
For a crispy, charred finish, switch the oven to broil for the last 5 minutes of cooking.
Once the chicken is beautifully golden and slightly crispy on the edges, remove it from the oven and let it rest for 5 minutes.
Using a sharp knife, shave thin slices off the stack, just like a classic gyro rotisserie.
Warm your pita bread in a dry skillet or directly over an open flame for a slight char.
Spread a generous spoonful of tzatziki sauce on the pita.
Layer on the sliced chicken, red onions, tomatoes, shredded lettuce, and feta cheese.
Wrap it up, take a bite, and enjoy!