In a bowl, gently mix ground beef, egg, almond flour, onion & garlic powder, Worcestershire (or tamari), salt, and pepper. Divide into 4 even portions and shape into patties with a slight dimple. Chill 10 minutes if you have time.
Heat a large skillet over medium-high heat. Add a little oil if needed. Sear patties 3–4 minutes per side until browned. Remove to a plate.
Reduce heat to medium, add butter, and sauté mushrooms until browned and moisture evaporates (4–6 minutes). Scrape up browned bits.
Pour in beef broth to deglaze. Stir in Dijon and tamari, simmer 4–6 minutes. Add heavy cream and bring to a gentle simmer.
Make a slurry of xanthan gum (1/8 tsp) in a tablespoon of cold water; whisk into gravy a little at a time until you reach desired thickness. Simmer briefly.
Return patties to the skillet, spoon gravy over, and simmer 4–6 minutes until cooked through. Finish with a knob of butter and chopped parsley. Serve hot.