Soak the lentils in approx. 350 ml water overnight.
The next day then bring 200 ml of vegetable stock with the lentils and the soaking water to a boil in a stockpot. Season with the bay leaf, salt and pepper and simmer for 40 minutes.
In the meantime, clean the leek and cut into rings, dice the remaining vegetables and onions. Heat the oil in the pan and sauté the onions until translucent. Then add the vegetables and the pressed garlic. Fry everything well (approx. 10 minutes). Deglaze with the remaining vegetable stock and simmer for another 5 minutes.
Add the vegetables to the lentils and remove the bay leaf. Season the soup with sugar, balsamic vinegar, mustard and thyme and serve hot.