Position an oven rack in the center of the oven, and preheat to 350 degrees F. Stir together the sugar, butter, cinnamon, cumin and 1/2 teaspoon salt in a small bowl.
Toss the sweet potatoes with the cornstarch in a large mixing bowl. Transfer half the sweet potatoes to a 6-quart slow cooker. Spoon half the sugar-butter mixture on top. Add the remaining potatoes and sugar-butter mixture. Cook, covered, on low until the sweet potatoes are tender but not mushy, about 4 hours.
Meanwhile, spread the walnuts out on a baking sheet, and bake until very golden brown, 10 to 12 minutes. Let cool, then roughly chop.
When the sweet potatoes are tender, gently stir in the lemon juice. Transfer the potatoes to a shallow serving dish, and let sit for 10 minutes (the sauce will thicken as it sits). Toss the potatoes in the sauce to coat. Sprinkle with the chopped walnuts, pomegranate seeds and parsley.
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