Clean the champignons. If the mushrooms are large, cut them in half. Cut the onion and carrot into small cubes.
Pour water into the pan. Add oil, salt, pepper, sugar, bay leaf, vinegar, onion and carrot. Bring to a boil and cook for a few minutes. Then pour the mushrooms and cook for another 5 minutes.
Transfer the champignons to a sterilized jar.
Tip: it is better to choose mushrooms of the same size, because this way they will be evenly saturated with marinade!