Preheat the oven to 350 F. Line a rimmed baking sheet with parchment or aluminum foil.

Warm the oil in a medium saucepan over moderate heat.  Add the onion and garlic, and cook, stirring occasionally, until the onion is softened and translucent but not brown.  Set aside.

Put the breadcrumbs in a small bowl, and drizzle the milk over them, stirring to moisten. Set aside.

Put the beef and pork in a large mixing bowl, breaking up any large hunks.  Add the salt, fish sauce, eggs, mustard, parsley, and 1/3 cup ketchup.  Add the onion and garlic. Using your hand, squeeze the milk from the breadcrumbs; then add the breadcrumbs to the meat mixture (discard the milk).  Holding one hand in a claw shape, press it down into the ingredients, and briskly stir with your hand to mix evenly.  When the meat and seasonings are uniformly mixed, pick up the mixture and turn in over in the bowl, and briefly mix again (Turning it over helps to ensure that no ingredient settles to the bottom and clumps there.)

Transfer the mixture to the prepared baking sheet, and use your hands to pat and shape it into an approximately 9 by 5-inch loaf.  (If you find that the mixture is sticking to your hands, rinse them well and leave them slightly wet; the moisture will keep the meat from sticking).  Brush the loaf evenly with the remaining 1/4 cup ketchup.

Cook for 45 minutes to 1 hour, or until a thermometer inserted into the center of the loaf reaches 155 F to 160 F.  Cool for at least 20 minutes before slicing.