Finely chop the onions and mushrooms into crescents. Mix together the butter and olive oil, heat them and use them to stew the vegetables (onions and mushrooms).
Cut the potatoes into small cubes and add them to the vegetables. Add 5 cups of hot water, with the broth cube dissolved in it, as well as the salt, finely chopped garlic and black pepper.
Let the soup boil on low heat under a lid for 25-30 min. Mix the cream with 1 cup of the soup, to equalize the temperature, then add it to the mushrooms.
Boil for 5 minutes.