Saute the mushrooms and onions in half of the butter for 5 minutes over medium heat.
Mix the beef and chicken stock; add mushrooms and onions. Add toasted sesame seeds and remaining butter; heat, stirring, until butter melts. Add salt. Cook over low heat for 10 minutes. Cool slightly. Stir in heavy cream. Do not allow to boil.
Serve warm or cold, garnished with chives. Add pepper as desired. Enjoy!