Wash and cut the vegetables and set aside.
Cut the steak into cubes and season with salt and pepper.
Heat 1 Tbsp of oil in a large skillet or Dutch oven over medium heat. Once hot, add the steak and brown all sides. It should only take 4 to 5 minutes. Remove and set aside.
Add the other 1 Tbsp of oil to the skillet and add the prepared vegetables (onions, mushrooms, and carrots).
Cook these for about 5 minutes, stirring occasionally.
Add the whiskey and cook one minute. Add the beef stock, stir, and let cook for 4 to 5 minutes. The liquid should reduce a bit.
While it is cooking, mix the cornstarch with 1/4 cup of water to make a slurry. Pour the slurry into the skillet and stir until it thickens, about 2 minutes.
Stir in the Greek yogurt and mustard. Add the reserved beef and stir until piping hot, about 1-2 minutes.
Add salt and pepper as needed. Pour the mixture on top of the prepared butter noodles and sprinkle with parsley. Enjoy!