Gently combine the beef, cream, grated onion, egg, tablespoon of Dijon, Worcestershire, garlic powder, salt, and pepper. Stop mixing once the seasonings look evenly distributed so the meat remains loosely textured rather than tightly packed.

Shape the mixture into six patties and brown them in the avocado oil in a large, hot skillet for 2-3 minutes on each side, then transfer them to a plate to keep warm. Look for a caramelized surface and a steady sizzle, leaving the centers to finish cooking in the gravy.

Cook the mushrooms in the butter in the same skillet over medium-high heat for 4-5 minutes. They should soften and release their moisture, with browning developing where the edges contact the pan.

Pour 1/2 cup of the beef broth into the skillet and loosen the browned deposits with a spatula. The broth should pick up color as the flavorful pan crust releases.

Return the patties and their plate juices to the skillet, add the tomato paste, remaining broth, and teaspoon of Dijon, then cover and simmer over low heat for about 10 minutes, until the beef is cooked through. Keep the liquid gently bubbling and check the centers with a meat thermometer rather than relying on the browned exterior.

Transfer the cooked patties to a clean plate and, if using, gradually whisk the optional 1/2 teaspoon xanthan gum into the pan sauce. Allow the thickener to hydrate before judging the consistency, looking for gravy that coats a spoon without becoming gummy.

Spoon the mushroom gravy over the beef patties. The sauce should settle around the meat and cling lightly to its browned surface.