In a large mixing bowl, combine ground beef, breadcrumbs, diced onion, beaten egg, salt, and pepper. Mix gently until just combined to keep the patties tender. Shape the mixture into 6–8 oval patties and set them aside to rest.

Heat an oven-safe skillet over medium heat. Carefully place the patties into the skillet and brown them for 3–4 minutes on each side until golden. Remove the patties and set them aside.

In the same skillet, melt butter over medium heat and stir in sliced onions, sautéing until softened. Sprinkle in flour, stirring constantly for about 2 minutes until lightly golden. Gradually whisk in beef broth and heavy cream, allowing the mixture to thicken.

Return the browned patties to the skillet, nestling them into the gravy. Cover tightly and transfer to a preheated oven at 350°F. Bake for 45 minutes, then uncover and bake for an additional 15 minutes.

Once baked, remove from the oven and let it rest for a few minutes before serving hot with sides like mashed potatoes.