Put half of the butter in a large saucepan and heat over medium heat. When the butter has melted, add the meat. Stew it for about 3 minutes, stirring occasionally. Then add the onion and cook for another 2-3 minutes. Transfer the contents to another container and set aside. Add the remaining half of the butter and add the sliced mushrooms. Cook until soft and then add the mustard and beef broth. Return the meat and onion and reduce the heat. Cook for about 20 minutes, stirring occasionally. You can cover the pot with a lid, but not tightly closed. Once the meat is soft, add the sour cream and cook for another 4-5 minutes. If necessary, thicken the dish with starch dissolved in a little cold water. Serve with boiled rice or short pasta sprinkled with parsley on top.