1. Melt the oil in a large sauce pan over medium heat. Add the onion, bell pepper, and garlic and cook, stirring occasionally, until the onion is tender but not browned, about 4 minutes. Add the cumin and stir well.
2. Add 8 cups water, lentils, tomato, 1 teaspoon salt, and 1/2 teaspoon pepper to the pot. Bring to a simmer.
3. Stir in the sweet potatoes (make sure they are submerged).
4. Reduce the heat to medium-low and simmer, uncovered, stirring occasionally, until the lentils and sweet potatoes are tender, about 45 minutes.
5. While the soup cooks, add hot water as needed to keep the soup from becoming too thick.
6. During the last few minutes, stir the coconut milk and heat it without boiling.
7. Season to taste with salt and pepper. Ladle the soup into bowls and serve.