Slice the beef sirloin against the grain into thin, bite-sized pieces, about 1/8 inch thick. Lightly dust the beef with salt and freshly ground black pepper, then set aside.

Prepare the gluten-free noodles, potatoes, or rice according to package instructions. Drain the noodles or rice if needed and set aside for serving later. I always make sure the noodles are slightly undercooked as they'll soak up some sauce later.

Heat the olive oil in a large skillet over medium-high heat. Add the finely diced onion and sliced cremini mushrooms. Cook, stirring occasionally, until the vegetables are tender and the onions have softened, about 5–7 minutes.

Add the sliced beef (from Step 1) to the skillet with the sautéed onions and mushrooms. Sear for about 5 minutes, stirring occasionally, until the beef is lightly browned.

Add the butter or ghee to the skillet and stir until fully melted, coating all the ingredients. Sprinkle arrowroot starch evenly over the meat mixture and stir continuously for 1–2 minutes until no dry powder remains and the mixture is slightly thickened.

Pour the beef broth, Worcestershire sauce or coconut aminos, Dijon mustard, garlic powder, the remaining salt and black pepper into the skillet. Stir well to combine and bring everything to a simmer. Let the sauce cook for 3–5 minutes until it reduces slightly and thickens. I like to taste and adjust seasoning here for flavor balance.

Remove the skillet from the heat and stir in the room temperature sour cream or dairy-free alternative until fully blended and creamy. Serve the stroganoff mixture over the cooked noodles, potatoes, or rice (from Step 2) and sprinkle with fresh chopped parsley, if desired.