Preheat a large frying pan over medium high heat and add in the olive oil. Add the beef and season with salt and pepper. Cook to brown on all sides, stirring occasionally, about 5 minutes. Remove from heat.
Add the mushrooms to the same pan and cook to brown on all sides, stirring occasionally, about 5 minutes. Deglaze the pan with the vinegar.
Add the beef back to the pan along with the stock. Cook 2 minutes and remove from heat.
Stir in the sour cream. Stir until well combined.
Serve over the cauliflower rice and top with the parsley.