In a medium mixing bowl or shallow baking dish, combine the Greek yogurt, olive oil, lemon juice, garlic, oregano, cumin, salt, and black pepper. Whisk until the mixture is well blended and smooth. The marinade should have a creamy consistency and a bright, herbaceous aroma.

Place the chicken thighs into the marinade and ensure each piece is thoroughly coated. Use your hands or tongs to gently massage the marinade into the chicken. Cover and refrigerate for a minimum of 1 hour, though up to 24 hours is recommended for optimal flavor and tenderness.

Preheat the oven to 400°F (200°C). If using a vertical spit designed for home ovens, prepare it according to the manufacturer’s instructions. Alternatively, slice a large onion in half and place it flat-side down on a foil-lined baking sheet. Insert two long metal skewers vertically through the onion to create a homemade spit base.

Thread the marinated chicken thighs onto the skewers, rotating each piece slightly as you stack them to form a layered, even tower. Pack the pieces tightly to help them cook evenly and hold their shape during roasting.

Transfer the assembled skewer to the preheated oven. Bake for 1 to 1½ hours, or until the outer edges are golden brown and the internal temperature reaches 165°F (74°C). Check periodically to ensure even browning. If needed, tent the top with foil to prevent over-browning.

Remove the chicken from the oven and allow it to rest for 10 to 15 minutes. Once rested, carefully slice thin layers from the stacked chicken, as one would with traditional gyro meat.

Serve the sliced gyro chicken in warm pita bread with tzatziki sauce, chopped vegetables, or as part of a rice bowl or roasted vegetable platter.