Season the beef strips with salt and pepper. Heat olive oil in a large skillet over medium-high heat. Sear the beef in batches for about 1 minute per side until browned. Transfer to a plate and set aside.

In the same skillet, melt 1 tablespoon of butter over medium heat. Add the diced onion and sauté for 3-4 minutes until softened. Add the garlic and cook for 30 seconds until fragrant.

Add the sliced mushrooms and cook for 5-7 minutes, stirring occasionally, until golden brown and all the liquid has evaporated.

Sprinkle the flour over the mushroom mixture and stir for 1 minute. Gradually pour in the beef broth while stirring constantly. Stir in the Dijon mustard and Worcestershire sauce. Bring to a gentle simmer.

Reduce the heat to low. Stir in the sour cream until the sauce is smooth and creamy. Do not let it boil after adding the sour cream.

Return the seared beef to the skillet and stir gently. Let everything warm through for 2-3 minutes.

Serve the beef stroganoff over cooked egg noodles. Top with a dollop of sour cream and a sprinkle of fresh parsley.