In a large bowl, combine ground beef, breadcrumbs, the beaten egg, diced onion, salt, and pepper. Mix gently with your hands until just combined.

Shape the mixture into 6 to 8 oval patties, ensuring they're evenly sized for consistent cooking.

Heat an oven-safe skillet over medium-high heat. Brown the patties for 3 to 4 minutes on each side until golden brown.

Remove the patties and set them aside, leaving the drippings in the skillet.

In the same skillet, melt butter with the drippings over medium heat. Sauté the sliced onions until softened.

Sprinkle flour over the onions, stirring continuously, then gradually whisk in beef broth and heavy cream to create gravy.

Nestle the browned patties back into the gravy, cover with foil, and bake at 350°F for 45 minutes.

Remove foil and bake for an additional 15 minutes.

Let rest for 10 minutes before serving with your favorite sides.