To a large (>12 cup) pot, add stock and green lentils; bring to boil. Reduce heat, cover and simmer for ~20 minutes.
Meanwhile, in large saucepan, heat oil over medium heat - fry onion, garlic, celery, potato, herb seasoning, salt and pepper. Stir occasionally, until potato is cooked through and onion is softened, about 10 minutes.
Once lentils have finished cooking (all the vegetable broth has been absorbed), add onion mixture to pot.
Using potato masher, mash soup until slightly thickened but still chunky. Add carrot, peas, corn and tomato paste; simmer, covered, until carrots are tender, about 10 minutes.
Lastly, stir in lemon juice.
To serve, ladle into bowls; garnish each 1 cup serving with a scant tablespoon of parsley.