In a large bowl, combine 1/2 teaspoon salt, cardamom and allspice. Add beef; turn to coat. In a large skillet, heat oil over medium heat; brown meat in batches. Transfer meat to a 3- or 4-qt. slow cooker. In the same skillet, cook onions until tender, 4-5 minutes. Add ginger, garlic, curry powder, cumin and remaining 1 teaspoon salt; cook 1 minute longer. Transfer to slow cooker., Cook, covered, on low until meat is tender, 4-5 hours. Stir in spinach; cook until heated through, about 30 minutes. Just before serving, stir in yogurt. Serve with rice and, if desired, additional yogurt and red onion.