Cook egg noodles according to package directions while preparing remaining ingredients. Drain and set aside—this keeps them separate so they don't overcook while you prepare the stroganoff skillet sauce.

Heat olive oil in a large skillet over medium-high heat until shimmering. Season beef sirloin with salt and pepper, then working in batches, sear 2-3 minutes per side until browned. Transfer to a plate without crowding the pan.

In the same skillet, sauté diced onion and sliced mushrooms for 5-6 minutes until they soften and golden brown. Add minced garlic and cook 1 minute more, stirring constantly to prevent burning. This builds aromatic foundation for your stroganoff sauce.

Stir in tomato paste, Worcestershire sauce, and Dijon mustard, cooking 1 minute until fragrant. Pour beef broth into the saucy beef stroganoff skillet, scraping browned bits from the bottom using a wooden spoon. Return beef to pan and simmer 5 minutes.

Whisk together sour cream and cornstarch, then slowly stir into simmering sauce to avoid curdling. Add paprika and taste for seasoning. Cook 2-3 minutes more until sauce thickens beautifully and coats the beef.

Toss cooked noodles with the stroganoff beef mixture until everything is coated evenly. Garnish with fresh parsley and serve immediately for best presentation and temperature.