In a saute pan over medium heat, melt butter. Add onions and mushrooms and cook until onions begin to turn translucent.
Add ground beef, garlic, thyme, salt, and pepper, breaking ground beef apart into small pieces. Cook until beef is browned and cooked through. Add flour and stir to coat beef, cooking for one minute.
Add beef broth and scrape to deglaze bottom of pan. Add egg noodles. Bring broth to a boil, cover with a lid, and reduce heat to low. Simmer until pasta is al dente. Most of the liquid should be absorbed.
Add sour cream and stir until incorporated. Remove pan from heat and allow to set for 5 minutes to allow sauce to thicken before serving.
Pasta skillet meals will continue to thicken as they stand - the starch from the pasta will help thicken your sauce, and the pasta will continue to absorb liquid. If your sauce looks thin, allow it to set, uncovered, until your sauce has thickened.
Optionally, garnish with fresh parsley for color.
Leftovers will reheat well the next day.
Add more sour cream as desired for a creamier sauce.