First, you want to cut the steak into thin strips. Then season lightly with salt and pepper.
Now, you want to quickly sear the beef strips over medium-high heat with a little oil until browned. Don't try to cook them all the way through you just want to add a little color then remove them. Cook in batches to avoid crowding the pan.
Turn heat down to medium high. Next add in and melt butter before adding in the onions and garlic. Cook until browning and try to deglaze the pan to get all the bits up from the steak.
Then toss in the flour and cook within the other ingredients for at least 1 minute.
Next, you want to pour in the beef broth.
Allow the broth and flour to come together before adding in the sour cream and mustard. Stir to combine everything, the sour cream will melt and come together with the other ingredients just give it time.
Bring to simmer ,then reduce heat to medium low. Once it thickens and is creamy which will take 3-5 minutes, add salt and pepper to taste.
Then, toss the beef into the sauce and allow to simmer for a minute or two together and you're done. Serve over the top of egg noodles and enjoy.