Place your chicken thighs in a large bowl and add the Greek yogurt and all of the spices. Mix well until the meat is evenly coated.
Add the olive oil, apple cider vinegar, and the two cloves of crushed garlic. Cover everything and refrigerate for at least 12 hours.
When you are ready to cook the gyros, move the oven shelf to the lowest setting and preheat to 350°. Then, line a baking sheet with parchment paper.
Cut the onion in half, remove the skin, and place it on the baking sheet with the flat side down. Poke the 3 bamboo sticks into the onion, forming a triangle shape, to create the base where you will skewer the pieces of meat. The triangle should span most of the width of the onion.
Begin creating your gyros by threading each chicken thigh so that they all fit together perfectly. Be sure not to press them too tightly together. They should be lightly resting on top of each other. Keep the excess marinade; you will need it later.
To close your "tower," cover the other half of the onion with foil and stick it on top of the meat like a hat. Remember to not apply too much pressure. Making the tower too compact may result in an overcooked outside but raw inside.
Bake for approximately 65 minutes.
Halfway through cooking, brush the surfaces of the gyros with the leftover marinade. You can repeat this step several times, as long as you leave the gyros in the oven for at least 10 minutes after the final brushing. This will ensure the juices from the raw chicken in the marinade can cook properly.
If you want to be 100% sure it's fully cooked, we recommend using a kitchen thermometer. Once the internal temperature reaches 170° it's safe to eat.