Add the chicken to a large bowl and drizzle with olive oil, yogurt, tomato paste, lemon juice, garlic, oregano, paprika, cinnamon, cumin, salt and pepper. Mix well and marinate for 2-6 hours, or ideally overnight.
Preheat oven to 400°F. Line a baking tray with parchment paper and add the marinated chicken without overcrowding. Add the onion wedges around the chicken, drizzle with more olive oil, and sprinkle with sumac.
Bake for 37-40 minutes.
Make the cucumber sauce by mixing the yogurt, cucumber, dill, shallot, lemon juice, salt and pepper.
Remove the sheet pan from the oven, top with the cucumber sauce, fresh herbs, more sumac, and lemon juice.
