In a large bowl, gently combine the ground beef, breadcrumbs, beaten egg, diced onion, salt, and pepper. Mix until just blended to avoid tough patties.

Form the mixture into 6 to 8 oval-shaped patties. Allow them to rest as you prepare the gravy base.

Heat an oven-safe skillet over medium heat. Brown the patties in the skillet for about 3 to 4 minutes per side until they achieve a golden crust. Remove the patties and set them aside.

In the same skillet, add the butter and sliced onions. Sauté until the onions are translucent and fragrant.

Sprinkle the flour over the sautéed onions and stir for about 2 minutes until lightly browned. Gradually whisk in the beef broth and heavy cream until smooth and blended.

Nestle the browned patties into the gravy mixture. Cover the skillet tightly and bake in a preheated oven at 350°F (175°C) for 45 minutes. Remove the cover and bake for an additional 15 minutes.

Once finished, remove the skillet from the oven and let it rest for about 10 minutes before serving.