To prepare the soup, melt one ½ tbsp. of butter over medium heat in a large pot. Add the onion and cook until soft, about 10 minutes, stirring occasionally. Raise the heat, add the porcini mushrooms and button mushrooms, chicken stock and water and bring to a boil. Reduce heat to medium low and continue to simmer uncovered for 30 minutes, until mushrooms are tender. Remove from heat and let cool.