Combine olive oil, lemon juice and zest, garlic, smoked paprika, ground cumin, ground coriander, crushed red pepper flakes, salt, pepper, turmeric, and cinnamon in a gallon-sized resealable plastic bag. Seal the bag and gently squeeze ingredients together until they are well blended. Add chicken thighs to the bag, squeeze out most of the air, seal, and refrigerate for at least 2 hours, or up to 8 hours.

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Preheat the oven to 425 degrees F (220 degrees C). Lightly grease a 12×18-inch baking sheet. Spread onion slices over the bottom; add chicken thighs on top.

Bake on the center rack of the preheated oven, about 30 minutes. Switch on the broiler and move rack closer to the broiler. Broil chicken until a light char develops, 2 to 3 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C). Garnish with parsley.