Slice the sirloin steak across the grain into 1/2-inch strips. Season evenly with salt and black pepper. Heat 1 tablespoon olive oil and 1 tablespoon butter in a large heavy skillet over high heat. Sear beef strips quickly for 1 to 2 minutes per side until browned but still tender pink inside. Transfer beef to a plate and set aside.

In the same skillet, melt the remaining 1 tablespoon of butter over medium heat. Add sliced cremini mushrooms and diced yellow onions. Sauté for 5-6 minutes until mushrooms are deeply caramelized and onions are soft. Add minced garlic and cook for 1 minute until fragrant.

Sprinkle flour over the mushrooms and onions; stir constantly for 1 minute to cook out the raw flour taste. Gradually pour in the beef broth while scraping up all the golden browned bits from the pan bottom. Stir in Worcestershire sauce and Dijon mustard. Simmer for 3-4 minutes until the sauce thickens to a luscious gravy.

Reduce heat to low. Stir in room temperature sour cream until smoothly incorporated and silky. Return the seared beef strips and any resting juices back to the skillet. Simmer gently for 2 minutes to heat through (do not boil to keep sour cream silky smooth).

Spoon generous heaps of creamy beef stroganoff over a bed of warm buttered egg noodles. Garnish with chopped fresh parsley and serve immediately.