Soak 10 wooden kebab sticks in water for at least 30 minutes. In a large bowl, toss the chicken pieces together with the cumin, salt, sugar, and pepper flakes. Thread onto the skewers (you’ll have between 8 and 10 total) and refrigerate for one hour or up to 3 days.
When you’re ready to make the dish, fill a kettle of water and bring to a boil. Place the bulgur in a medium bowl with a pinch of salt. When the water boils, pour 1 ½ cups over the bulgur. Cover with a plate and set aside.
About 30 minutes before grilling, remove chicken from the refrigerator. Light the grill (if using charcoal, wait until the coals are glowing and ashy on top; if using gas, preheat to high on one side and medium on the other)
Cut two of the lemons in half and place in a small bowl with the onion. Toss with 1 teaspoon olive oil and a pinch of salt.
Place the chicken skewers over the coals and arrange the lemon halves (cut side down) and the onions on the outer edges of the coals (or on the cooler side of the grill).
Sear the chicken for about 2 minutes, flip and cook for another 2-4 minutes. Remove from heat, transfer to a platter, and rest while the lemons and onions continue cooking.
Meanwhile, make the tahini sauce - pound the garlic clove into a paste by mashing it with the side of your knife with a pinch of salt on the cutting board (or use a mortar and pestle). Stir together the garlic with the tahini, lemon, remaining teaspoon of olive oil and another pinch of salt, then pour in hot water until you’ve thinned the sauce to consistency you like (I like thick but pourable). Taste and adjust seasoning. Transfer to a small bowl and set aside.
Cook the onions until tender and charred at the edges and the lemon until softened and browned on the cut side, another 5-8 minutes.
Taste the bulgur and add more salt if needed. Pile it onto a platter and place the skewers on top. Squeeze one of the lemon halves over the chicken skewers. Arrange the remaining 3 lemon halves on the platter with the onions, and serve with the tahini sauce alongside