Season and High-Heat Sear the Beef: Pat the sliced sirloin steak completely dry with paper towels. Season evenly with kosher salt, black pepper, and garlic powder. Heat 1 tablespoon olive oil and 1 tablespoon butter in a large deep cast iron skillet over high heat until sizzling hot. Add the beef strips in a single layer and sear undisturbed for 1.5 to 2 minutes until deeply browned on the edges. Flip and cook for 1 minute more (the meat should be medium-rare; do not overcook). Immediately transfer seared beef and juices to a clean plate and set aside.

Sauté Mushrooms and Onions: Reduce the skillet heat to medium. Melt the remaining 2 tablespoons butter in the residual pan drippings. Add the sliced cremini mushrooms and chopped yellow onion. Cook for 6 to 8 minutes, stirring occasionally, until the mushrooms are deep golden-brown and the onions are soft, translucent, and caramelized.

Aromatize and Build the Roux: Add minced garlic to the skillet and sauté for 60 seconds until fragrant. Sprinkle the all-purpose flour over the mushrooms and onions, stirring constantly for 1 minute to cook out any raw flour taste.

Simmer and Thicken the Savory Gravy: Slowly pour in the beef bone broth while whisking or scraping the bottom of the skillet to dissolve all the flavorful browned bits (fond). Stir in Worcestershire sauce and Dijon mustard. Bring the liquid to a gentle boil, then reduce heat to low and simmer for 4 to 5 minutes until the gravy thickens into a glossy, velvety consistency that coats the back of a spoon.

Temper and Fold in the Sour Cream: Remove the skillet completely from the heat (or keep on the lowest possible heat setting). Ladle 2 to 3 tablespoons of hot gravy into your bowl of room temperature sour cream and stir to temper. Gently fold the tempered sour cream into the pan gravy until completely smooth, silky, and pale golden in color.

Reintroduce Beef & Serve over Egg Noodles: Return the cooked beef ribbons and all their resting juices back into the skillet. Stir gently over low heat for 1 to 2 minutes just until warmed through. Serve immediately spooned generously over hot buttered curly egg noodles, crowned with finely chopped fresh Italian parsley and extra cracked black pepper!