Cut steak against the grain into ¼-inch thick strips, trim off excess fat, and sprinkle with 1 ½ teaspoons salt.
In a large skillet, heat 1 tablespoon olive oil over high heat.
When working in small batches, cook the beef by resisting the urge to move the meat around (similar to our Philly Cheese Steak recipe).
You are looking for a nice brown sear, wait about 1–2 minutes. Flip the beef and continue cooking until both sides are nice and browned.
Remove from pan and repeat with remaining meat. When done, set the meat aside. Note: If the pan is too dry, add more olive oil.
Add the butter to the same pan. Once melted, add the onion and mushrooms and cook until the onion softens and the mushrooms release all their liquid and begin to brown, about 5 minutes. Add the garlic and sauté for 1 minute more.
Add the flour to the onion mushroom mixture for 1 minute. Slowly pour in the beef stock a little at a time and stir until fully incorporated.
Add thyme, salt, and pepper and cook, stirring often, until thickened, about 5 minutes.
Add the Worcestershire sauce, sour cream and Dijon mustard and stir until the sauce is combined.
Bring the sauce to a gentle boil and simmer until the sauce thickens. Add the meat back to the sauce along with fresh parsley and serve with noodles or mashed potatoes.
Enjoy!