In a bowl, combine minced garlic, ground coriander, paprika, black pepper, salt, chili flakes, olive oil, tomato paste, and milk to create the marinade.
Place the boneless chicken thighs in the marinade, ensuring they are well-coated.
Cover the bowl and let it marinate in the refrigerator for at least 2 hours or overnight for better flavor.
Preheat the oven to 400°F (200°C).
Place a large skillet or heavy-based pan that you can use in the oven over high heat.
Add the chicken thighs and cook for a minute on each side, until they are nicely browned.
Transfer the skillet to a preheated oven and bake the chicken thighs for 14-15 minutes, until the core temperature reaches 165°F (74°C).
Once cooked, remove the chicken from the oven and let it rest for a few minutes.
Slice the chicken shawarma and return it to the skillet, and cook for a few more minutes.
Serve it on a plate or wrapped in pita a bread, and enjoy with your favorite toppings and sauces.