In a medium bowl, combine the yogurt, olive oil, paprika, cumin, cinnamon, red pepper flakes, lemon zest, lemon juice, salt, pepper and garlic.
Thread the chicken onto metal skewers, folding if the pieces are long and thin, alternating occasionally with the red onions. Be sure not to cram the skewers. (Note: You’ll need between 6-8 skewers.) 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. To grease the grill, lightly dip a wad of paper towels in vegetable oil and, using tongs, carefully rub over the grates several times until glossy and coated. Grill the chicken kebabs until golden brown and cooked through, turning skewers occasionally, 10 to 15 minutes. Transfer the skewers to a platter and serve.