Slice The Beef: Begin by slicing the beef tenderloin or sirloin into thin strips. This guarantees fast and consistent cooking of the meat.

Season: Season the beef strips with salt and black pepper.

Heat The Oil And Butter: In a large skillet, heat the olive oil and butter over medium-high heat.

Sear The Beef: Add the beef strips to the skillet and sear them until browned on all sides. This should take about 5-7 minutes. After taking the steak out of the skillet, set it aside.

Sauté The Onions And Garlic: In the same skillet, add the chopped onion and cook until softened, about 5 minutes. Add the minced garlic and continue cooking for one more minute.

Add The Mushrooms: Stir in the sliced mushrooms and cook until they release their moisture and start to brown, about 7-10 minutes.

Deglaze The Pan: Pour in the brandy or cognac, scraping up any browned bits from the bottom of the skillet. Simmer for two to three minutes to gently decrease.

Add The Broth And Seasonings: Stir in the beef broth, Dijon mustard, and Worcestershire sauce. Let the mixture simmer for 5 minutes.

Incorporate The Sour Cream: Reduce the heat to low and stir in the sour cream. Stir until a creamy, smooth sauce is achieved.

Prepare The Flour Mixture: If you prefer a thicker sauce, mix 2 tablespoons of flour with 1/4 cup water to form a slurry.

Add To The Sauce: Gradually stir the flour mixture into the sauce, cooking until it thickens, about 3-5 minutes.

Return The Beef: Add the seared beef back into the skillet, stirring to coat with the sauce. Cook for an additional 5 minutes to ensure the beef is heated through.

Cook The Noodles: While the beef and sauce are simmering, cook the egg noodles according to the package instructions. Drain and set aside.

Serve: Plate the egg noodles and top with the beef stroganoff. Garnish with fresh parsley.