Heat olive oil in a large Dutch oven over medium-high heat. Add the onion; sauté for 3 minutes or until tender. Add the turmeric and the next 6 ingredients (turmeric through garlic); sauté for 1 minute. Add water and next 4 ingredients (water through diced tomatoes); bring to a boil. Reduce heat; simmer 1 hour.
Reserve 2 cups lentil mixture. Place half of remaining mixture in blender; process until smooth. Pour pureed soup into a large bowl. Repeat procedure with other half of remaining mixture. Stir in reserved 2 cups lentil mixture. Garnish with chopped tomatoes and a cilantro sprig, if desired.