Mix the sugar, cinnamon, turmeric, nutmeg, allspice, 1 tsp. salt, and 1/2 tsp. pepper in a bowl. Add the chicken and coat evenly. Set aside.
Melt the butter in a large skillet set over medium heat. Add the onion;
cook, stirring often, until softened, about 5 minutes. Stir in the
walnuts and continue cooking, stirring almost constantly, for 2 minutes,
until the nuts are lightly browned.
Stir in the pomegranate molasses, then scrape the contents of the skillet into a 6-quart slow cooker. Spoon the spiced chicken on top of the cooked onion mixture, scraping any spices that cling to the bowl into the slow cooker.
Cover and cook on low until the chicken is tender and cooked through, 3-1/2 to 5 hours. Season to taste with salt, stir in half of the pomegranate seeds, and garnish with the remainder. Serve.