Melt butter in a large skillet over medium heat. Add mushrooms, onion, and garlic to the skillet and sauté until tender (about 8-9 minutes). Remove from skillet and set aside on a plate.
Brown the ground beef in the same skillet over medium-high heat until no longer pink (about 7 minutes), breaking it into small crumbles as it cooks. If you're not using extra lean ground beef, drain off the excess fat.
Add thyme, rosemary, 1 cup of the broth, salt, and Worcestershire sauce (and optional brandy/sherry/Marsala, if using). Bring to a boil, then reduce heat to low. Cover and simmer for about 10 minutes.
In a small bowl, whisk together the remaining ½ cup of broth with the flour until completely smooth. Stir the slurry into the beef mixture. Add the onion and mushroom mixture back to the skillet and bring to a boil, stirring constantly. Boil and stir for 1 minute, then reduce heat to low.
Stir in the sour cream and keep over low heat, just until warmed through (do not boil). Taste and season with additional salt and pepper, if necessary.
Serve over cooked egg noodles or rice and garnish with fresh parsley, if desired.