Mix the steak. In a small bowl, combine ground beef, almond flour, Worcestershire, Dijon, minced onion, garlic powder, salt, and pepper.
Mix gently with your hands or a fork until just combined. Don’t overwork it.
Shape the patty. Form into an oval patty about 3/4 inch thick. Press a slight dimple in the center so it cooks evenly.
Sear the steak. Heat a small skillet over medium heat.
Add butter or oil. Sear the steak 3–4 minutes per side until well browned. Transfer to a plate; it will finish in the gravy.
Sauté vegetables. In the same skillet, add 1/2 tbsp butter.
Add mushrooms and onions with a pinch of salt. Cook 3–4 minutes, stirring, until softened and lightly browned.
Build the gravy. Pour in beef broth, Worcestershire, Dijon, thyme, and pepper. Stir well, scraping up browned bits from the pan.
Simmer 2–3 minutes.
Thicken the sauce. Sprinkle a tiny pinch of xanthan gum evenly over the surface while whisking. Start very small; it thickens quickly. If not using xanthan, keep simmering 3–4 more minutes to reduce to a silky consistency.
Finish cooking the steak. Nestle the steak and any juices back into the gravy.
Spoon sauce over the top. Simmer 3–5 minutes, until the center reaches your preferred doneness. Target 160°F (71°C) for well-done.
Taste and adjust. Add salt if needed and more pepper to taste. If the gravy gets too thick, splash in a bit more broth.
Serve. Plate with cauliflower mash or a quick green side.
Spoon the mushroom-onion gravy over the steak. Garnish with parsley.