Combine ground beef, chopped mushrooms, egg, onion powder, garlic powder, thyme, salt, and black pepper in a mixing bowl. Mix well until just combined. Form the mixture into four evenly sized patties, about 3/4-inch thick.
Heat a skillet over medium-high heat and add butter. Sear the patties for 3-4 minutes per side until browned and cooked through. Transfer the cooked patties to a plate and keep warm.
In the same skillet, add the remaining finely chopped mushrooms and cook for 2-3 minutes until softened. Add minced garlic and cook for 1 minute.
Pour in beef broth, scraping the bottom of the skillet to release any browned bits. Bring to a simmer. Reduce heat to medium, and let simmer for 5-7 minutes.
Add fresh thyme and heavy cream, stirring well until the gravy reaches a smooth, velvety consistency. Taste and adjust seasoning with additional salt or pepper if needed.
Return the Salisbury steaks to the skillet to reheat slightly or serve separately, spooning the mushroom gravy generously over each steak.