Pat the sirloin strips dry with paper towels and season generously with salt and pepper on all sides.

Heat 1 tablespoon of olive oil in a large skillet over high heat until shimmering and nearly smoking. Add half the beef in a single layer — do not crowd the pan. Sear for 60-90 seconds per side until deeply browned. Remove to a plate and repeat with remaining beef. Set aside.

Reduce heat to medium. Add butter and the remaining 1 tablespoon olive oil to the same skillet. Add the diced onion and cook for 3-4 minutes, stirring occasionally, until softened and translucent.

Add garlic and cook for 30 seconds until fragrant.

Add the sliced mushrooms in a single layer. Cook undisturbed for 3 minutes to allow browning, then stir and cook for another 3-4 minutes until deeply golden and all moisture has evaporated.

Sprinkle the flour over the mushroom mixture and stir to coat. Cook for 1 minute to remove the raw flour taste.

Pour in the beef broth, Worcestershire sauce, and Dijon mustard. Stir well, scraping up all the browned bits from the bottom of the pan. Bring to a gentle simmer and cook for 4-5 minutes until the sauce has thickened slightly.

Remove the skillet from heat completely. Stir in the room-temperature sour cream until fully incorporated and the sauce is smooth and creamy. Taste and adjust seasoning with salt and pepper.

Add the seared beef back to the pan along with any resting juices. Stir gently to coat the beef in the sauce.

Serve immediately over cooked egg noodles. Garnish with fresh chopped parsley.