Slice the sirloin steak into thin 2-inch strips and season with sea salt and black pepper.
Heat avocado oil in a large skillet over medium-high heat and sear the beef strips until browned, about 2 minutes per side, then remove from the pan and set aside.
In the same skillet, add the sliced mushrooms and diced onions, sautéing for 5-6 minutes until the mushrooms are golden and the onions are translucent.
Add the minced garlic to the skillet and cook for 30 seconds until fragrant.
Pour in the beef bone broth and Dijon mustard, using a wooden spoon to scrape up any browned bits from the bottom of the pan.
Reduce the heat to low and whisk in the Greek yogurt until the sauce is smooth and creamy, being careful not to let it boil.
Return the beef to the skillet and toss gently to coat in the mushroom sauce.
In a separate small pan, lightly sauté the cauliflower rice for 3-4 minutes until tender.
Serve the beef and mushroom sauce over the cauliflower rice and garnish with freshly chopped parsley.