Preheat your oven to 350°F (175°C). In a large mixing bowl, combine ground beef, seasoned breadcrumbs, garlic powder, salt, black pepper, egg, and Dijon mustard. Mix until well combined.
Shape the beef mixture into four evenly sized patties, about 4 ounces each. Heat vegetable oil in a large skillet over medium heat and sear the patties for 4-5 minutes on each side until they are nicely browned.
Transfer the browned patties to the oven and bake them for approximately 10 minutes or until they reach your desired level of doneness.
In the same skillet, reduce the heat to medium-low and add the diced onion. Sauté for about 3 minutes until softened, then add minced garlic and cook for an additional minute.
Pour in the beef broth and let it simmer for 2-3 minutes. Gradually stir in the heavy cream, then incorporate the Parmesan cheese, horseradish, thyme, salt, and pepper. Allow the sauce to thicken slightly for about 5 minutes.
Once the patties are done baking, return them to the skillet and spoon the creamy sauce over them. Cook for 2-3 minutes to meld the flavors.
Serve the Salisbury steaks hot, drizzled with the sauce and garnished with chopped chives.