In a large skillet, heat the oil over medium heat. Add the chicken, cornstarch, salt, and pepper to a large bowl. Toss to coat the chicken completely.

Cooking in batches, add the chicken to the skillet and brown on all sides. Transfer to a plate, and repeat with the remaining chicken.

Drain off all but 1 tablespoon of fat, and add the onion to the pan. Cook until softened and then stir in the garlic. Cook for 1 minute longer. Transfer the chicken and onion mixture to the slow cooker.

In a small bowl, whisk together the turmeric, garam masala, cumin, cayenne pepper, tomato sauce, and ginger. Pour this mixture over the chicken mixture.

Add the squash, raisins, chicken stock, cinnamon stick, and peppers to the slow cooker. Stir to combine. Cover and cook on Low for 6 hours or on High for 4 hours.

10 minutes prior to serving, stir in the half and half and spinach. Season to taste. Serve with rice and chopped cilantro, if desired.