In large bowl, gently mix ground beef, breadcrumbs, beaten egg, diced onion, salt and pepper until just combined. Form into 6-8 oval patties about ¾ inch thick.
Preheat oven to 350°F. Heat large oven-safe skillet over medium-high heat. Brown hamburger steaks 3-4 minutes per side until golden. Remove and set aside, leaving drippings in pan.
Add butter to skillet drippings. Sauté sliced onions for 3-4 minutes until softened and lightly golden.
Sprinkle flour over onions and cook 2 minutes, stirring constantly.
Gradually whisk in beef broth and cream until smooth. Add garlic powder, salt, and pepper to taste.
Nestle browned steaks back into gravy. Cover tightly with foil and bake for 45 minutes.
Remove foil and bake 15 minutes more until gravy thickens.
Let rest 10 minutes before serving for perfect consistency.
Notes
Use 80/20 ground beef for best flavor and moisture during baking.