Heat oil in a large saucepan over medium heat; cook onion and carrot, covered, stirring occasionally, for 10 minutes or until softened. Add cumin, coriander and chilli; stir to coat. Add lentils and stock; bring to the boil. Reduce heat; simmer, covered, for 35 minutes or until lentils and carrots are soft.
Meanwhile, to make coriander pesto; blend or process coriander, garlic and pistachios until finely chopped. With motor operating, add oil and juice in a thin, steady stream until combined. Season to taste.
Blend or process soup until smooth. Return soup to pan with the water; bring to the boil. Season. Serve soup drizzled with pesto, topped with coriander leaves.
Lentils will thicken on standing; if necessary add a little extra water or stock to thin the soup.