Cut 2 onions into large pieces. Clean the soup greens and wash very thoroughly. Peel carrots and celery root. Put aside the peelings and cut greenery. Cut soup greens and celery greens into large pieces.
Heat the oil in a large pot. Add the onion pieces, soup vegetables in pieces, peels set aside, 1 bay leaf and peppercorns to the pot. Fry for about 5 minutes. Add approx. 1.5 litres of water.
Salt, bring to the boil and simmer at low heat for about 1 hour. Pour stock through a sieve (better: sieve with inserted, moistened passing cloth/etamine) into another pot. Reduce the stock to approx. 1 litre or, if necessary
fill up with water. Season to taste with salt and a pinch of sugar. Wash and clean the zucchini and cut them into small cubes. Wash spring onions and cut into fine rings. Peel garlic and press it through a garlic press.
Cut out 4 circles (4 cm Ø) from each slice of bread. Grate cheese finely. Heat oil in a pot. Sauté the zucchini, spring onions and garlic. Deglaze with broth, add 1 bay leaf, bring to the boil.
Let simmer for about 5 minutes. Line baking tray with baking paper. Place bread circles on top and sprinkle with cheese. Bake in the preheated oven (electric cooker: 200 °C/ circulating air: 175 °C/ gas: level 3) for approx. 5 minutes.
Remove the laurel from the soup. Season soup with salt and pepper and serve. Add 3 cheese croutons to the soup.