PASTA: If serving over pasta, start by bringing a large pot of water to boil. Once boiling, add in a generous amount of salt (about 1/2 tablespoon). Add in the pasta and cook according to package directions. Drain when cooked through (I try to time pasta cooking to when the meat sauce will be done).
ONIONS, GARLIC, AND BEEF: Preheat a large, deep, cast-iron skillet or pot over medium-high heat. Add the 1 tablespoon olive oil and once shimmering, add in the 1 cup diced yellow onion and 2 teaspoons minced garlic. Stir until onion is translucent and beginning to turn golden, about 3-4 minutes; reduce the heat if needed. Press onions and garlic to the side and increase the heat to high. Add the 1 pound ground beef to the middle of the pot. Let stand, undisturbed for 30 seconds and then flip and break up. Let the other side stand for about 30 seconds and then continue to cook the beef, breaking it up into small pieces until no longer pink. Season the beef to preference with salt and pepper. If anything is burning/sticking to the pot, add an extra drizzle of olive oil or slightly reduce the heat.
MUSHROOMS: Once the beef is cooked through (no longer pink) add in the 8 ounces of sliced mushrooms and the 1 tablespoon butter. Sauté for 3-4 minutes, stirring constantly. Add the 1/2 teaspoon dried thyme and stir for 30 seconds. Then mix in the 3 tablespoons flour and stir. Cook for 1 minute to remove the floury taste.
GRAVY: Add 1 cup beef broth and scrape up any browned bits that may have formed at the bottom of the skillet as you stir. Stir in the 1 cup heavy cream, 1 tablespoon Worcestershire sauce, 1 teaspoon Dijon mustard, and season everything to taste with salt and pepper. Add in the 1/2 teaspoon beef bouillon powder. Reduce the heat to low and simmer for 3-5 minutes or until thickened, stirring frequently.
FINISHING: Remove pan from the heat and stir in the 1/3 cup sour cream (Note 1). Stir to combine everything and season to taste as needed. Serve over cooked noodles, rice, or mashed potatoes and top with fresh parsley.
Note 1: Sour Cream: Be sure to add sour cream off the heat and stir slowly to avoid curdling.