In a large bowl, combine sumac, 2 tablespoons olive oil, minced garlic, lemon juice, lemon zest, cumin, thyme, salt, and black pepper. Whisk until smooth.
Add chicken thighs and turn to coat fully. Press marinade into skin. Rest at room temperature 20 to 30 minutes, or refrigerate up to 4 hours.
Preheat oven to 425 degrees Fahrenheit. Toss sliced onions with remaining 1 tablespoon olive oil and a pinch of salt. Spread evenly across a 9×13 inch baking dish.
Place marinated chicken skin-side up over the onions. Roast 35 to 40 minutes until skin is deep golden and internal temperature reads 165 degrees Fahrenheit.
Remove from oven. Dust extra sumac over chicken skin. Scatter fresh parsley on top. Rest 5 minutes before serving.
