Heat the olive oil in a large saucepan or Dutch oven. Lightly brown the onions in the oil. Add the tomatoes, chili pepper, lentils, cumin and basil to the pan along with the water. Bring to a boil, then reduce heat to medium-low and simmer for about 20 minutes, or until the lentils are tender.

When the lentils have softened, use a stick blender to puree the soup. Season to taste with salt and pepper. For a special touch, put sour cream in a squirt bottle, and squeeze a spiral onto the top of each bowl of soup, and garnish with a sprig of fresh basil.