Heat oil in a stock pot. Add onion and salt, and cook for 5 minutes or until softened. Add coriander and cook for 1 minute. Add carrots, potato, and salt. Pour in the tea stock, bring to a simmer, and cook uncovered for 25–30 minutes or until the vegetables are tender. Use a stick blender or pour into a blender and blend until smooth. Season to taste. Serve warm, topped with croutons and cilantro.