In a large bowl, combine ground beef, breadcrumbs, beaten egg, diced onion, salt, and pepper until just mixed. Form the mixture into 6-8 oval patties, each about ¾ inch thick. Let the patties rest for about 10 minutes.

Preheat your oven to 350°F. Heat an oven-safe skillet over medium-high heat and add a splash of oil or butter. Carefully place the patties in the hot skillet, cooking for 3-4 minutes per side until golden brown.

In the same skillet, melt butter over medium heat, then add the sliced onion, sautéing until soft and golden, about 5 minutes. Sprinkle in the flour, stirring constantly for about 2 minutes to create a roux.

Gradually whisk in the beef broth and heavy cream, stirring until the mixture is smooth and thickens.

Return the browned patties to the skillet, cover with aluminum foil, and bake for 45 minutes. Uncover and continue to bake for an additional 15 minutes.

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