Clean mushrooms and cut into smaller pieces, if necessary. Peel onion and cut into strips. Bring plenty of water with bay leaves and 1 teaspoon of salt to a boil, add mushrooms and onion strips and blanch for 2-3 minutes. Lift out with a slotted spoon and place into prepared preserving jars. Fill jars with fresh cold water so that water is about 2 cm higher than mushrooms (approximately 3/4 inch). Cover jars with a cloth and place into a large pot. Fill pot with water up to jars' rims and simmer for about 2 hours. Remove from heat and cool, seal well and keep in a cool, dark place. Serve as needed.