In a large mixing bowl, combine the ground beef, breadcrumbs, beaten egg, diced onion, salt, and pepper. Mix gently until just combined, being careful not to overwork the meat. Shape into 6-8 oval patties and let them rest while heating the skillet.

Heat an oven-safe skillet over medium-high heat. Add the formed patties and cook for 3-4 minutes on each side, until golden-brown. Remove and set aside.

In the same skillet, reduce heat to medium and add butter. Sauté sliced onions until softened and lightly golden, about 5-7 minutes.

Sprinkle flour over sautéed onions and stir for about 2 minutes. Gradually whisk in beef broth and heavy cream. Cook for 3-5 minutes until gravy thickens.

Return browned patties to skillet, cover, and bake at 350°F for 45 minutes.

Remove from oven, let rest for 10 minutes, and serve hot with gravy.