Prep the beef: Freeze the steak for 20–30 minutes to firm it up. Slice thinly against the grain. Pat dry and season with salt and pepper. This helps with browning.

Sear in batches: Heat a large skillet over medium-high. Add 1 tablespoon oil and half the butter. Sear the beef in batches for 1–2 minutes per side until browned but not fully cooked. Transfer to a plate and repeat with remaining beef.

Sauté the vegetables: Reduce heat to medium. Add the remaining oil if needed. Cook the onions for 3–4 minutes until softened. Add mushrooms and a pinch of salt; cook 5–7 minutes until they release moisture and brown. Stir in garlic for 30 seconds until fragrant.

Build the sauce: Sprinkle in paprika, then pour in the beef broth. Scrape up browned bits. Stir in Dijon and Worcestershire. Simmer 3–4 minutes to reduce slightly.

Make it creamy: Reduce heat to low. Whisk in sour cream and heavy cream until smooth. Do not boil once dairy is added to prevent curdling.

Thicken (optional): If you want a thicker sauce, sprinkle 1/8–1/4 teaspoon xanthan gum evenly over the surface while whisking constantly. Give it a minute to set.

Return the beef: Add the seared beef and any juices back to the skillet. Simmer on low for 2–3 minutes until the beef is tender and just cooked through. Adjust salt and pepper.

Finish and serve: Stir in chopped parsley. Serve over cauliflower mash, zucchini noodles, or shirataki noodles. Spoon extra sauce over the top.