01 - Prepare egg noodles or rice according to package directions, drain, and set aside.
02 - Heat 1 tablespoon vegetable oil and 1 tablespoon butter in a large skillet over medium-high heat. Add beef strips in batches, searing each side for 1 to 2 minutes until browned but still slightly pink inside. Remove from skillet and set aside.
03 - Reduce heat to medium. Add remaining oil and butter to skillet. Sauté chopped onions for 2 to 3 minutes until softened, then add minced garlic and cook for an additional 30 seconds.
04 - Add sliced mushrooms to the skillet, cooking and stirring occasionally until golden brown and most moisture has evaporated, about 5 to 6 minutes.
05 - Sprinkle flour evenly over vegetables and stir continuously for 1 minute to incorporate.
06 - Pour beef broth into the skillet, scraping up any browned bits from the bottom. Stir in Dijon mustard and Worcestershire sauce. Simmer for 2 to 3 minutes until the sauce thickens slightly.
07 - Lower heat to low, then stir in heavy cream and sour cream until the sauce is smooth. Return beef strips along with their juices to the pan and simmer gently for 3 to 4 minutes to heat through. Season with salt and freshly ground black pepper to taste.
08 - Plate the stroganoff over cooked noodles or rice and garnish with chopped fresh parsley.