1. Cut steaks into cubes and season generously with salt and pepper. 
2. Heat a pan to a high heat and add butter or olive oil (or both). Cook steak in 2 batches for approx 1 min to sear each side and get some nice colour. Make sure not to overcrowd the pan. 
3. Remove steak once browned and set aside on a plate (no paper towel - you want the juices).
4. Reduce the heat in the pan to medium. 
5. Add sliced onion and sliced mushrooms to pan at medium heat to saute. The mushrooms will release liquid and then it will evaporate - may take about 10 mins. 
6. Add dijon mustard and tomato paste and cook off for 30 seconds to release the flavour. 
7. Add broth powder and water and mix to combine with the rest of the ingredients. Cook down for 5 mins to reduce slightly. 
8. Add sour cream to a bowl and add 1/2 cup of the sauce from the pan to the sour cream and mix to temper. If you add the sour cream directly to the pan it may split, so this helps the sour cream to come to temperature prior to adding it to the hot pan. Mix in the tempered sour cream. 
9. Finally add the steak and juices to the pan. 
10. Cook for a few minutes altogether to cook steak to medium (or how you like it). Sprinkle with chopped parsley for freshness and serve immediately with brown rice, mashed potato or cauliflower rice. Enjoy!

Notes

If the sauce is a bit thin you can thicken with your flour of choice by mixing 1 tbsp flour with 4-5 tbsp of the hot sauce and mix back in. Or you could cook for an additional 10 mins prior to adding the steak back in to cook off more liquid.