Bring a large pot of salted water to a boil. While it heats, slice the beef across the grain into thin 1/4-inch strips. Pat dry with paper towels and season with salt and pepper. Slice mushrooms and chop onion and garlic.

Add egg noodles to boiling water and cook until al dente. Drain, toss with a small pat of butter, and keep warm.

Heat 1 tablespoon butter and olive oil in a large skillet over high heat. In two batches, sear beef strips for about 1-2 minutes per side until browned and an instant-read thermometer inserted into the thickest strip reads 145°F (63°C) for medium-rare. Transfer to a plate; do not wipe pan.

Reduce heat to medium. Add remaining 1 tablespoon butter to the skillet. Sauté onion until translucent, about 3 minutes. Add mushrooms and cook until golden and their liquid evaporates, 5-6 minutes. Stir in garlic and cook 30 seconds.

Sprinkle flour over the mushroom mixture and stir for 1 minute. Gradually whisk in beef broth, scraping up browned bits. Add Worcestershire and Dijon mustard. Bring to a simmer and cook until thickened, 2-3 minutes.

Remove skillet from heat. Stir in sour cream until fully incorporated. Return beef and any accumulated juices to the pan, warming gently over low heat (do not boil). Adjust seasoning with salt and pepper.

Divide warm egg noodles among bowls, ladle stroganoff over the top, and garnish with fresh parsley. Serve with extra black pepper at the table.