Heat a large heavy-bottomed pot over medium heat, add the olive oil and heat through. Add the onions, leeks and garlic and sauté until the onion is soft and translucent.
Add all of the spices (coriander, cumin, paprika, turmeric, cinnamon, and white pepper) and stir to combine. Let it cook, stirring frequently, until fragrant, about 2 minutes. Add the tomato paste and stir to combine.
Add the bell pepper, carrots, sweet potatoes, lentil and chickpeas, tomatoes, vegetable stock and 2 cups of water to the pot then stir to combine.
Bring the mixture to a boil then reduce the heat to medium-low and let simmer about 1 hour and 30 minutes, until the lentils are cooked through and the sweet potatoes are tender. Add additional water ½ cup of water at a time if the soup becomes dry.
Remove the pot from heat and season with kosher salt. Stir in the orange zest and half the cilantro. Ladle the soup into warmed soup bowls. Top remaining cilantro and serve with warmed naan bread on the side.