Combine 2 cups of beef stock, 2 tablespoons of Worcestershire sauce, 2 tablespoons of cornstarch and 1 tablespoon of tomato paste in a small bowl and set aside for now
Add 2 tablespoons of butter into a large skillet over medium heat
Once the butter has melted, add the thinly sliced beef, season generously with salt and black pepper and sear in both sides
Transfer the beef onto a plate and set aside for now
Add the remaining 2 tablespoons of butter into the pan and use a wooden spoon or stiff spatula to work up any brown bits
Add the onions, mushrooms and garlic and cook for 5-7 minutes or until softened
Add the beef broth mixture, season with salt and black pepper and bring to a simmer
Once the sauce has thickened a bit, stir in 3/4 cup of sour cream and the seared beef
Garnish with finely chopped parsley and add additional salt and black pepper, if necessary
Serve hot over egg noodles or mashed potatoes