Heat olive oil in a large skillet over medium-high heat.
Add the beef and cook until browned on all sides, about 5 minutes.
Remove the beef from the skillet and set it aside.
In the same skillet, add the onion and garlic and cook until softened, about 3-4 minutes.
Add the mushrooms and cook for an additional 5 minutes, or until they release their moisture and start to brown.
Add the beef broth, sour cream, Dijon mustard, and Worcestershire sauce to the skillet and stir well to combine.
Add the beef back to the skillet and stir to combine.
Reduce the heat to low and let the mixture simmer for 10-15 minutes, or until the sauce has thickened.
Meanwhile, rice the cauliflower by pulsing it in a food processor until it resembles rice.
Steam the cauliflower rice in a separate pot for 5-7 minutes, or until tender.
Season the beef stroganoff with salt and pepper, to taste.
Serve the beef stroganoff over the cauliflower rice and garnish with fresh parsley.