Cut chicken into strips or chunks. Place in a large bowl.
Drizzle with olive oil, yogurt, tomato paste, lemon juice, garlic, thyme, paprika, cinnamon, cumin, and a couple pinches salt and pepper.
Mix really well and marinate for at least 2-6 hours, but ideally overnight.
When you're ready to eat, remove chicken from fridge and let it sit at room temp for 30 minutes.
Preheat oven to 400°. Line a baking tray with parchment paper and add the chicken to the tray; don't overcrowd the tray. Cut onions into wedges and toss around the chicken on the tray. Drizzle with more olive oil and sprinkle with sumac.
Pop into the oven for 37-40 minutes.
Make the cucumber sauce by mixing all ingredients with a couple pinches salt and pepper. Mix well.
Remove the sheet pan from the oven, then top with a few dollops of the spread, lots of fresh herbs (parsley, dill, or mint would be great here!), diced cucumber, another dusting of sumac, and fresh lemon juice.
