1.Melt butter in a large soup pot over medium-high heat; cook mushrooms in butter with 1 pinch salt until the mushrooms give off their juices; reduce heat to low. Continue to cook, stirring often, until juices evaporate and the mushrooms are golden brown, about 15 minutes. Set aside a few attractive mushroom slices for garnish later, if desired. Mix onion into mushrooms and cook until onion is soft and translucent, about 5 more minutes.