Start by drying the meat with paper towels. Place it in a medium bowl. Coat the steak with the flour and lots of salt and pepper. Set aside.
In a large pan, heat 1/4 cup of the butter until melted. Add the shallot and mushrooms (if using), and saute until the shallot is translucent. Place the beef in the pan and cook until browned. Add 2 Tablespoons of butter to the pan if it gets dry. Stir in the garlic, and saute until fragrant.
Pour the brandy into the pan, and let it reduce until there are only a couple of tablespoons left. Add the beef broth, bay leaf, mustard, thyme, salt, and pepper. Let it simmer for at least 35 minutes and up to 1 hour to tenderize the meat.
Meanwhile, cook the egg noodles (or rice) according to package directions. Stir in 2 Tablespoons of butter and couple pinches of salt.
When the beef is done simmering, stir in the half of the sour cream. Taste, and add more sour cream, 2 Tablespoons at a time, until it's to your desired consistency. I usually use the whole amount. Cook for about 5 more minutes to heat. Garnish with parsley, and serve over noodles or rice.