In a large pot, sauté the onion, celery and carrot until translucent.
Season with salt and pepper and add boiling water - 1,5L is an estimate. Add however much your pot takes.
Add the lentils and bring to a rolling boil. Cover and let cook for an hour, adding more boiling water as necessary.
This is the secret step! Trim the turnip and carrots and cut them and the leeks and celery into smaller pieces. Add to the bowl of a food processor. Add the parsley and blitz on high speed until the veggies are becoming a pulp. I often do this in two batches and I add some cold water through the spout of the food processor to assist with the pulping.
Peel the potato if you prefer and chop into small blocks.
Once the lentils have softened, add the vegetable pulp and the blocks of potato and bring to a simmer, stirring frequently.
Add the tomato paste, vinegar and miso paste (if using).
Cover the pot and let the soup simmer for an hour to an hour and a half, stirring frequently and adding more boiling water if some of the water cooks away.
Check the seasoning of the soup, adding salt or pepper as needed. The soup needs quite a lot of salt I find.
Serve the soup piping hot with croutons on top and crusty bread on the side.