Pick over lentils and wash in several changes of cold water. Drain.
Put lentils in a large pot with 8 cups water, spring onion, garlic if used, olive oil and parsley.
Bing to a boil, cover pan and simmer on low heat for 1 hour or until lentils are soft.
Put 1/4 cup water and flour in a screw top jar, seal and shake until thoroughly combined. This prevents lumps forming. pour this gradually into the boiling soup, stirring constantly until thickened slightly.
Add vinegar, sea salt and pepper to taste. Return to the boil and boil gently for 5 minutes. Serve with wedges of fresh lemon to squeeze onto individual portions and a fresh salad if wished.