Line a loaf pan or rimmed baking pan with foil. This will make it easier to lift the meatloaf out when it's done. Alternatively, use a meatloaf pan with a perforated insert.

Heat the oven to 350 F.

In a medium bowl, whisk the milk and eggs together; add the oats and let the mixture stand for 5 minutes.

Meanwhile, in a large bowl, combine the ground beef, ground pork, finely chopped onion, Worcestershire sauce, salt, pepper, garlic powder, onion powder, and half of the ketchup. Mix gently with your hands or a large spoon.

Add the oat, egg, and milk mixture and stir until well blended. Avoid overworking the mixture.

Pack the meatloaf into a foil-lined loaf pan, foil-lined baking pan, or a meatloaf pan with an insert.

Bake the meatloaf for 1 hour, then remove it to a rack.

Combine the remaining ketchup with the brown sugar and mustard and spread it over the top of the meatloaf.

Return the meatloaf to the oven and bake for 15 to 20 minutes longer.