Take the chicken off the grill and cover with aluminum foil. Let the chicken rest for 10 minutes.
To bake the chicken, place the chicken in a single layer of a baking dish. Roast the boneless chicken thighs for about 20 minutes, until the temperature reaches 165 °F (with an instant read thermometer). The thighs roast quickly so keep an eye on them.
Take the chicken out of the oven and cover with aluminum foil. Let the chicken rest for 10 minutes.
Now, put together the chicken gyros with spicy feta. In a pan, heat the flatbread for a couple of minutes. Once they are warm, place it on a plate, add a dollop of spicy feta and spread it. Then add the cucumber, tomato, chicken, onions, mint leaves, and a dollop of tzatziki sauce. And Voila! you have super delicious chicken gyros with spicy feta.