In a small bowl, combine the lemon zest, lemon juice, salt, pepper and garlic yogurt, olive oil, paprika, cumin, cinnamon, red pepper flakes.
Thread the chicken onto metal skewers, add the onion pieces along the way in between the chicken pieces.Place the kebabs on a baking sheet lined with aluminum foil. Spoon or brush the marinade all over the meat, coating well. Cover and refrigerate at least eight hours or overnight.
Preheat the grill to medium-high heat. Grease the grill by lightly dipping a paper towel in vegetable oil and, carefully rub over the grates several times until glossy and coated. Grill the chicken kebabs until golden brown and cooked through, turning skewers occasionally, about 10 to 15 minutes. Transfer the skewers to a plate and serve warm!