Heat oil in dutch oven over med high heat. Add leeks, and saute 5-7 minutes. Stir in tomato paste, peanut butter, harissa, garlic, tumeric, and cumin. Cook 1 min stirring constantly.
Add sweet potatoes, broth, and 1.5 cups of water and simmer over med low heat covered 20-25 minutes or until sweet potatoes are tender.
Blend mixture with immersion blender, adding more water if soup is too thick. Bring soup back to simmer and stir in spinach leaves and serve with lemon wedges if desired.
I use a whole quart of broth (4 cups), so only add 1 cup of water. However I think adding just broth over water gives it more flavor. I tend to add more spinach, as well as not blending it. I like texture. I serve with brown rice, roasted cauliflower and browned fennel. Together its divine!