If using wooden skewers, soak them in water for at least 30 minutes to prevent burning during grilling.
In a large bowl, mix all marinade ingredients: yogurt, lemon juice, garlic, tomato paste, olive oil, paprika, cumin, cayenne pepper, salt, and pepper.
Add the chicken cubes and mix thoroughly so that every piece is coated with the marinade.
Cover and refrigerate to marinate for a minimum of 5 hours, ideally overnight (maximum 24 hours).
Preheat the oven to 220 °C (425 °F) or heat the grill (or pan).
Thread the chicken cubes onto the skewers (you can alternate with pieces of onion or bell pepper if desired).
Oven Baking: Place the skewers on a baking sheet lined with parchment paper or hang them over the edge of the sheet. Bake for 20-25 minutes, turning halfway through, until cooked (internal temperature 74 °C (165 °F)) and beautifully golden brown.
Grilling: Grill over medium heat for 4-6 minutes on each side until cooked through.
Serve the skewers immediately. Traditionally, they are served with Lebanese rice, French fries, fresh salad, and especially with a generous amount of homemade Toum garlic sauce. They can also be served in pita bread as a wrap.