Slice your sirloin thinly against the grain. Dust with flour evenly; this will help the sauce thicken while keeping the beef tender. Set aside while you prep the vegetables.
Follow package instructions for the egg noodles. Drain well and set aside. This keeps them from getting soggy when mixed with the sauce later.
In a large skillet over medium-high heat, warm olive oil. Add diced onions and mushrooms, cooking until tender, about 7 minutes. Remove from the skillet and set aside.
Add butter to the same skillet. Once melted, add the beef strips and cook for 4–5 minutes until browned. Do not overcrowd the pan to ensure proper searing.
Return the onions and mushrooms to the skillet. Pour in beef broth and Worcestershire sauce, simmering 2–3 minutes until slightly thickened.
Stir in sour cream and Dijon mustard, blending until smooth. Add the cooked noodles and toss to coat evenly. Season with salt and pepper to taste, and serve hot.