In a large mixing bowl, add the ground beef, diced onion, quick oats, milk, eggs, Worcestershire sauce, sea salt, and garlic powder. Mix gently until ingredients are evenly distributed, taking care not to overwork the meat. Let the mixture sit at room temperature for 10 minutes to allow the oats and seasonings to hydrate and the flavors to meld.

Preheat the oven to 375°F (190°C). Line a loaf pan or a rimmed baking sheet with aluminum foil for easy cleanup. Shape the meat mixture with your hands into a loaf and place it into the prepared pan, pressing gently to form an even shape.

Spread an even layer of ketchup over the top of the formed loaf. Bake in the preheated oven for about 45 minutes, or until a thermometer inserted into the center registers 165°F (74°C).

Take the meatloaf out of the oven and transfer it to a cutting board or leave it in the pan. Let it rest for 10 minutes before slicing; this helps the juices redistribute so the slices hold together. Serve warm.