In a large mixing bowl mix together the ground beef, egg, grated yellow onion, gluten free bread crumbs, minced garlic, spicy brown mustard, worcestershire sauce, sea salt, dried basil and ground pepper. I use my hands, but you can use a large spoon also.

Place your 12-inch frying pan over medium high heat and add the 2 tablespoons of olive oil.

Press the beef mixture into six even patties and place them in the frying pan. Fry them until one side is browned and then flip to finish cooking. The patties should be at least 160 degrees in the center to be completely cooked. Once the patties are cooked remove them from the frying pan to a platter.

Add the unsalted butter to the frying pan on medium high heat and let melt. Add the yellow onion, mushrooms and minced garlic. Cook down the mixture until the onion is translucent and the mushrooms are cooked.

Sprinkle the gluten free flour over the onion mixture and let that soak up the liquid. Add the heavy cream, beef broth, sea salt, black pepper and dried oregano to the pan. 

Bring this mixture to a simmer and stir for 5-7 minutes until the gravy is thickened.

Add the salisbury steaks back to the pan and let them heat back up for 2-3 minutes. 

Serve this over mashed potatoes or with steamed mixed vegetables.