Step 1

Heat oil in a large skillet over medium-high heat until sizzling hot, about 1 minute. Add beef and sauté until browned on all sides, about 3-4 minutes. (If using leftover roast beef, add it in and cook for 1-2 minutes instead, until starting to warm). Set aside on a plate.

Step 2

In the same skillet, add butter and melt over medium-high heat until it starts to bubble, about 1 minute. Add mushrooms, onion, and garlic and sauté until golden brown and tender, about 3-4 minutes. Add flour and mix until well combined, about 1 minute.

Step 3

Add beef broth, heavy cream, sour cream, and mustard. Mix well until smooth and bring the mixture to a simmer, about 2-3 minutes.

Step 4

Return the cooked beef to the pan and season with salt and pepper. Stir well and let it simmer over medium heat until heated through and the sauce thickens to a desired consistency, about 3-4 minutes.

Step 5

Garnish with parsley and serve warm with pasta (I like to serve with egg noodles).To cook egg noodles: Bring a large pot of salted water to a boil over medium-high heat. Add egg noodles and cook according to package directions, about 6 minutes. Drain well and set aside.