In a 6-quart slow cooker combine sweet potatoes, carrots, and broth. Cover and cook on low for 8 to 10 hours or on high for 4 to 5 hours. Use a slotted spoon to transfer vegetables to a large bowl, reserving cooking liquid.
Meanwhile, in a skillet, cook leeks in olive oil over medium heat about 8 minutes.
Mash potato mixture until smooth, adding cooking liquid if needed to mash. Add milk, butter, honey, harissa, and salt. Mash until light and fluffy. Sprinkle with cooked leeks and sesame seeds before serving.