Heat oil in large pot or Dutch oven. Add onion, carrot, celery, garlic and red pepper flakes and cook over medium heat for approximately five minutes or until vegetables are fragrant and have softened.
Using an immersion blender, blend the soup until smooth, or leave some chunks if desired. Alternatively, you can transfer some of the soup into a blender to puree it.
Add extra broth to thin it out if necessary. Add lemon juice, salt and pepper and stir. Taste and adjust seasoning according to your personal taste preference.