Prepare the noodles or rice: In a large pot, bring a generous amount of salted water to a boil. Add the egg noodles or rice and cook according to package instructions until al dente. Drain and keep warm.
Sear the beef: In a large skillet, heat the olive oil over medium-high heat. Add the sliced beef, season with salt and pepper. Sear the beef for 2-3 minutes until browned but not fully cooked. Remove from the skillet and set aside.
Sauté the onions and garlic: In the same skillet, add the chopped onion and sauté for 2-3 minutes until soft. Then add the minced garlic and sauté for another minute until fragrant.
Cook the mushrooms: Add the sliced mushrooms to the skillet and cook for about 5 minutes, stirring occasionally, until golden brown and tender.
Combine flavors: Add the Dijon mustard and Worcestershire sauce, followed by the beef broth. Bring everything to a simmer and let it reduce lightly for about 5 minutes.
Finish the sauce: Reduce the heat and stir in the sour cream until well combined. Return the beef to the skillet and let it simmer for 3-4 minutes until the meat is cooked through and the sauce becomes creamy. Season with salt and pepper to taste.
Serve: Plate the warm noodles or rice and generously spoon the rich beef stroganoff mixture over the top.
Garnish: Sprinkle freshly chopped parsley over the dish for a pop of color and freshness.