1. Heat oil in a large heavy pot over medium-high heat. Add onion and cook, stirring, until softened. Add garlic, cumin, and paprika, and cook 1 minute, stirring. Add tomatoes, 1-quart water, tomato paste, cilantro, and 2 teaspoons salt. Bring to a simmer, cover, and simmer 30 minutes.
2. Turn off heat, remove and discard cilantro, and blend with hand blender until smooth. Season to taste with salt, pepper, lime juice, and cayenne. Serve hot or cold, with a spoonful of yogurt in each bowl (if using) and sprinkled with cilantro leaves.