In a 4 – 5 quart slow cooker, combine sweet potatoes, lentils, onion, jalapeño, and garlic. Add broth, undrained tomatoes, curry and ginger.
Cover and cook on low-heat setting for 7 – 8 hours or on high-heat setting for 3 1/2 – 4 hours.
Add the kale or spinach and salt and pepper during the last 30 minutes of cooking.
Check for seasoning and garnish with yogurt or sour cream, chili peppers and/or crushed red pepper if desired.