In a large bowl, combine the ground beef, gluten free breadcrumbs, egg, worcestershire sauce, dijon mustard, onion powder, garlic powder, salt, and pepper. Mix gently until just combined, then shape into 6 oval patties about 3/4 inch thick.
Heat a large skillet over medium-high heat. Sear the patties for 3 to 4 minutes per side until a deep brown crust forms. They don't need to be cooked through at this stage. Remove and set aside on a plate.
Reduce the heat to medium and melt the butter in the same skillet. Add the sliced mushrooms and onion. Cook for 6 to 7 minutes, stirring occasionally, until softened and golden brown.
Add the minced garlic and cook for 30 seconds until fragrant.
In a small bowl, whisk the cornstarch into the beef broth until dissolved. Pour the mixture into the skillet along with the worcestershire sauce and thyme. Stir well and bring to a simmer.
Return the patties to the skillet, nestling them into the gravy. Cover and simmer on medium-low heat for 12 to 15 minutes until the patties are cooked through and the gravy has thickened. Season with salt and pepper to taste.
Serve the salisbury steaks smothered in mushroom gravy alongside mashed potatoes or your favorite gluten free side.