Preheat your oven to 375°F (190°C). In a large mixing bowl, combine the ground beef, breadcrumbs, eggs, and diced onion. Mix everything together until the ingredients are evenly distributed and the mixture is well-combined.
Using your hands, shape the beef mixture into a loaf and place it in a large, oven-safe skillet or cast-iron pan. Gently press the top to create an even surface. Heat the skillet over medium-high heat and sear the meatloaf for 2-3 minutes per side, until a nice crust forms on the exterior.
Transfer the skillet to the preheated oven and bake the meatloaf for 45-55 minutes, or until the internal temperature reaches 165°F (75°C). About 10 minutes before the meatloaf is done, remove it from the oven and spread the ketchup evenly over the top. Return the meatloaf to the oven and continue baking until the glaze is set and the internal temperature is reached.
Once the meatloaf is cooked through, remove it from the oven and let it rest for 5-10 minutes. Slice the meatloaf and serve it hot, garnished with fresh parsley or your favorite herbs, if desired.
