Heat oil in a large Dutch oven over medium-high heat. Add onion; sauté 4 minutes. Add curry; sauté 1 minute. Add water and lentils; bring to a boil. Cover; reduce heat. Simmer 40 minutes or until lentils are tender.
Place 4 cups lentil mixture in a blender; process until smooth. Return mixture to pan. Add chopped basil, vinegar, salt, and tomatoes; cook until thoroughly heated. Garnish with basil, if desired.
Note: We particularly liked this soup made with red lentils. However, they're smaller, cook in a much shorter time, and need less liquid, so adjust accordingly.