In a large bowl combine the tomato paste, salt, pepper, smoked paprika, cumin powder, garlic, cinnamon, and olive oil. Mix well.: The air is perfumed with warm, roasted aromas as the spices blend together, and you should notice the paste turning slightly glossy as the olive oil incorporates. This emulsified mixture creates a clingy surface, which is why the chicken will pick up color and flavor so effectively. If the mixture seems too thick it may not coat evenly, so whisk vigorously to disperse the spices. One common mistake is under mixing, which can leave small pockets of concentrated spice that burn on the grill.
Add the cut up chicken to the mixture and stir to coat. Cover and refrigerate for at least 4 hours to overnight.: As the chicken rests, the marinade penetrates the meat, softening fibers and deepening flavor. You should notice the flesh becoming slightly darker and more uniformly colored after time in the fridge, a good sign that the spices have infused. The slow soak builds nuance, so don’t rush this step if you can avoid it. Avoid marinating on the counter, which risks food safety issues; always refrigerate to keep the texture tender and safe.
Preheat grill to medium heat and lightly spray with cooking spray.: Preheating ensures a steady, even sear that creates those sought after grill marks and caramelized edges on the chicken . Medium heat lets the exterior color without overcooking the interior, so you should hear a steady sizzle when the skewers hit the grate. If the grill is too hot, the exterior will char before the center cooks; too cool, and you lose that appealing caramelization.
Thread chicken and onion pieces onto sharp skewer sticks, alternating between the two and sliding each piece to the end of the skewer to build a "stack" of skewered ingredients.: Skewering tightly so pieces touch helps them cook evenly and keeps smaller items from rotating on the stick. The onion releases steam and sweet juices as it cooks, often sizzling and softening around the edges. A frequent error is uneven pieces, which leads to some bits overdone and some underdone, so aim for consistency in size.
Place the skewers on the grill and cook until chicken is no longer pink inside, about 12 minutes, turning every 3 minutes to cook the chicken evenly.: You will hear a steady, satisfying sizzle and see the surfaces deepen to a golden to lightly charred color. Turn the skewers periodically so each side gets contact with the hot grates, ensuring even browning. The meat should feel springy, and the juices should run clear when pierced. Rushing this step or leaving the skewers unattended can result in dry chicken or overly blackened edges, so stay attentive and use tongs for safe turning.
While the chicken cooks, wrap the grated cucumber in a kitchen towel and wring out the excess water.: Squeezing the grated cucumber removes excess moisture that would otherwise water down the sauce and make it runny. The towel will come away noticeably damp, and the cucumber left behind should be dense and slightly dry to the touch. Failing to remove the water is a common slip, and it results in a thin sauce that does not cling to the skewers.
Add the cucumber, Greek yogurt, garlic, salt, pepper, and 2 tablespoons of the chopped mint to a medium food processor. Blend until the mint is broken down, all the ingredients are well combined, and the mixture is smooth.: As the blade whirs you will smell the cool, herbal lift of mint and the clean tang of yogurt . The texture should be creamy and slightly thick, not soupy, so the sauce can be spooned alongside the skewers. If the sauce separates or looks watery, chill it briefly and stir; overprocessing can make it overly thin, so pulse to control consistency.
Garnish the skewers with fresh mint and serve immediately with the yogurt sauce on the side.: Serving right away captures the contrast between the warm, smoky chicken and the cool, herby sauce. The garnish adds a burst of fresh aroma and makes the plate look inviting. If you wait too long to serve, the grilled texture softens and the experience loses some of its impact, so plan to have everything at the ready before the skewers come off the grill.