Season steak with salt and pepper. In a large skillet, add some butter to sear the steak, add steak to the pan and cook for a few minutes until both sides are a little brown. Remove steak from pan.

In the same pan, add more butter and saute the onions and mushrooms for about 8-10 minutes.

Add flour to the onions and mushrooms, then whisk in the chicken broth. Add mustard and worcestershire sauce and sour cream. Add steak back into pan and cook on medium until the steak is medium inside.

Spoon over egg noodles or mashed potatoes for a hearty meal.