In a large bowl, mix together ground beef with all of the patty ingredients. Combine very well, until throughly mixed. Shape into oval like steak shapes. Set aside on a plate or baking sheet until ready to cook.
Heat cast iron skillet (at least 10″) over medium high heat. After hot, lower the heat to medium, then add olive oil to the pan, followed by the patties, in batches. Cook until browned on both sides, and the patties are cooked through. About 3 minutes on each side.
After the patties are completely cooked, remove them from the skillet and set aside. Add butter to the skillet, along with the mushrooms and a pinch of salt. Cook until mushrooms are softened, about 5- 7 minutes. Add in the diced onion and cook until soft, which should take about an additional 5 minutes.
Add in garlic and cook until fragrant. Stir in beef stock, Worcestershire sauce, and dijon mustard to warm through.
Add in sour cream and stir continuously until a sauce forms. Adjust seasoning to taste. Add the patties back to the skillet, and spoon the mushroom gravy over the gluten free salisbury steaks. Garnish with parsley and fresh cracked black pepper.