In a large, heavy soup pot, warm the oil over medium heat until it shimmers. Add the onion, and cook, stirring frequently, until tender, 5-10 minutes. Add the garlic, cumin, paprika, and salt, and cook, stirring, for 1 minute. Work in the tomato paste, then add the carrots, sweet potatoes, lentils, stock, water, chipotle, and tomatoes. Bring the soup to a boil, then reduce the heat to maintain a low simmer and cook, partially covered, until the lentils and sweet potatoes are tender but still holding a shape, 30-40 minutes. Taste, adding more salt or chipotle if you like.