In a large mixing bowl, gently combine ground beef with breadcrumbs, beaten egg, diced onion, salt, and black pepper. Form the mixture into 6 to 8 oval patties and set aside for 10 minutes.

Heat a large oven-safe skillet over medium-high heat and brown the patties for 3 to 4 minutes on each side. Remove from skillet and set aside.

In the same skillet, melt butter over medium heat, sauté sliced onions until soft, and then stir in flour to cook for 2 minutes.

Gradually whisk in beef broth and heavy cream, stirring continuously until the mixture thickens. Simmer for 3 to 5 minutes.

Preheat the oven to 350°F (175°C). Return the patties to the gravy, cover tightly with foil, and bake for 45 minutes, uncovering and baking for an additional 15 minutes.

Let the dish rest for 10 minutes after baking before serving with gravy spooned over each patty.