First, warm your skillet over medium-high heat and add the olive oil. Once it's shimmering, add the ground beef in a spread-out layer and let it sit briefly to develop browning—don’t stir too soon. After a few minutes, start breaking it apart using your spoon so the pieces are small and consistent. Then toss in your diced onion and garlic, sautéing until the onions become soft and luminous. This step builds the savory base for the sauce.
Add the sliced mushrooms next and sauté for about 5 minutes until their liquid evaporates and they begin to brown. Mushrooms release a lot of moisture, so letting that steam off before proceeding ensures your sauce isn’t watery. Once they’re tender and color is developing, sprinkle the flour over everything, stirring well. The flour cooks out the raw taste and gives your sauce its classic velvety thickness.
Slowly pour in the beef broth while scraping up any tasty browned bits from the bottom of the pan—they're flavor gold. Then pour in the whipping cream and add Worcestershire sauce, salt, and pepper. Bring the mixture to a gentle simmer, allowing the sauce to develop richness and thicken—about 5 minutes. Simmer gently; high heat could separate the dairy.
Remove the pan from heat and stir in the sour cream bit by bit, whisking constantly so it incorporates smoothly. Taste and adjust seasoning—sometimes it needs a touch more salt, pepper, or Worcestershire. Serve it over warm egg noodles, rice, or mashed potatoes. Sprinkle freshly chopped parsley or dill on top for a burst of color and freshness that balances the creaminess.