1 medium-size yellow onion, halved and cut into 1⁄4-inch/0.65 cm slices
In a small bowl, combine the cinnamon, coriander, ginger, pepper, nutmeg, allspice, and 2 tablespoons of the oil. Arrange the chicken on a plate or platter and spread both sides with the spice paste. Sprinkle both sides with salt. Cover and chill in the refrigerator for at least 4 hours, or overnight.
Preheat the oven to 450°F/230°F and let the chicken come to room temperature.
In a large, ovenproof skillet over medium-high heat, heat the remaining 2 tablespoons of oil. Sear the chicken without moving on one side until browned, 4 to 5 minutes. Turn the chicken, transfer the skillet to the oven, and roast until the chicken is cooked through, 8 to 10 minutes. Remove the skillet from the oven (careful—the handle of the skillet will be very hot), transfer the chicken to a plate, and let it rest, covered loosely with foil.
Meanwhile, place the skillet on the stovetop over medium heat and add the onion. Cook, stirring occasionally and scraping up any browned bits in the skillet, until the onion starts to soften, about 3 minutes. Lower the heat to low and cook until tender and wilted, about 7 minutes. Add salt to taste.
Arrange the chicken on plates or a platter and top with the onion. Serve hot, drizzled with any accumulated juices or pan drippings.