Heat a 1 tbsp oil in a large skillet over medium-high heat.
Season the sliced steak with salt and pepper and place it in the hot skillet, searing on all sides for 1 minute until browned. Sear in batches if necessary/skillet is not big enough to hold all the meat in an even layer.
Transfer the seared steak to a bowl and set aside.
Reduce the heat to medium and add the butter. Once melted, add the sliced onion, cooking for 1 minute before adding the sliced mushrooms. Add a pinch of salt and saute for 8-10 minutes, stirring occasionally, until all moisture has been released from the mushrooms and onions are soft and golden.
Add beef broth and scrape up any browned bits from the bottom of the pan.
Once the broth has thickened slightly, mix a few tablespoons of hot broth with sour cream in a separate small bowl. Add the mixture to the broth and vegetables and stir to incorporate.
Taste and season as necessary. Continue to cook and allow the broth to thicken, then add the steak back to the skillet and simmer until warmed and fully cooked through, about 4-5 minutes.
Serve over blanched zoodles or steamed cauliflower rice and garnish with chopped parsley and freshly cracked black pepper, if desired.
Serve immediately and enjoy!
Best stored in an air tight container in the fridge up to 5 days.