Bring a large pot of salted water to a boil and cook penne according to package directions until al dente. Drain and set aside.
Season beef strips with salt and pepper. In a large skillet over medium-high heat, melt 1 tablespoon of butter and sear beef until browned, approximately 2-3 minutes. Remove beef and set aside.
In the same skillet, melt the remaining 1 tablespoon of butter. Add sliced mushrooms and diced onions, cooking until softened and browned, about 5-7 minutes. Add minced garlic and cook for 1 minute until fragrant.
Sprinkle flour over the vegetables and stir constantly for 1 minute to cook out the raw flour taste.
Slowly whisk in beef broth, Worcestershire sauce, and Dijon mustard. Simmer for 3-5 minutes until the sauce thickens.
Reduce heat to low. Stir in the sour cream until smooth. Return the cooked beef and accumulated juices to the skillet.
Add the cooked penne to the skillet and toss to coat thoroughly in the sauce.
Garnish with fresh parsley and serve immediately.