1Add chicken, olive oil, greek yogurt, tomato paste, lemon juice, garlic, oregano, paprika, cinnamon, cumin, salt, and pepper to a large bowl. Mix well and marinate for 2-6 hours, or ideally overnight.
2Preheat oven to 400F (200C). Bring the chicken to room temperature for 30 minutes.
3Line a baking tray with parchment paper and add the chicken without overcrowding. Cut 2 red onions into small wedges and toss them around the chicken on the tray. Drizzle with more olive oil and sprinkle with 1 tbsp of sumac.
4Bake in the preheated oven for 37-40 minutes.
5Make the cucumber sauce by mixing 3/4 cup plain greek yogurt, 1 diced Persian cucumber, 3 tbsp finely chopped fresh dill, 1 small shredded shallot, 1/2 lemon juice, salt, and pepper.
6Remove the sheet pan from the oven. Top with cucumber sauce, fresh herbs, sumac, and fresh lemon juice.