In a large bowl, mix ground beef, breadcrumbs, grated onion, egg, Worcestershire sauce, garlic powder, salt, pepper, mustard powder, and paprika.
Shape into oval patties (about 4-6) and let them rest for 10 minutes.
Heat a large skillet over medium-high heat. Add a little oil and sear patties until browned on both sides (about 3-4 minutes per side).
Remove patties from the pan and set aside.
In the same pan, melt butter and add mushrooms. Cook until golden brown.
Add sliced onions and sauté until caramelized (about 5 minutes).
Sprinkle flour over the onions and stir for 1 minute.
Gradually add beef broth while stirring, followed by heavy cream and thyme.
Stir in Kitchen Bouquet if using.
Return the patties to the skillet and spoon gravy over them.
Simmer for 10-15 minutes until the gravy thickens and the patties are cooked through.
Serve hot with mashed potatoes, rice, or buttered noodles.