Cut boneless skinless chicken thighs into chunks. Place in a large bowl.
Drizzle with olive oil, 3 tbsp yogurt, tomato paste, juice of 1 large lemon, garlic cloves, oregano, paprika, cinnamon, cumin, and a couple pinches salt + pepper.
Mix really well and marinate for at least 2-6 hrs, but ideally overnight (makes a big difference!)
When you're ready to eat, remove chicken from fridge and let it sit at room temp for 30 mins.
Preheat oven to 400F. Line a baking tray with parchment or foil, and add the chicken to the tray a piece at a time (don’t overcrowd). Place onion around the chicken on the tray. Drizzle with more olive oil and sprinkle with the sumac.
Pop into the oven at 400F for ~25 mins.
Make the cucumber sauce by mixing ¾ cup yogurt, cucumber, dill, shallot, juice of half a lemon, a couple pinches salt & pepper. Mix well.
Top chicken and onions with dollops of the spread throughout, and lots of fresh herbs (parsley, dill, or mint).
Serve in warmed tortillas!