In a pot, combine butter and olive oil, and heat until the butter melts. Add the onions and cook over medium heat for 10-15 minutes, stirring occasionally, until they caramelize. Season with salt.
Add the mushrooms to the pot and sauté for an additional 5 minutes.
Pour in the beef broth and add thyme.
Cover the pot and let it boil for 20-30 minutes.
Turn off the heat, then stir in the mustard and cream. Serve and enjoy!