Preheat the oven to 350°F / 177°C / gas mark 4. Grease a loaf pan and set it aside.
Heat olive oil in a small skillet over medium heat. Saute onion until it's translucent but not brown. (About 3 minutes.)
Tip the onions, with the oil, into a large bowl and add the beef, oatmeal, egg, seasoned salt, ketchup, and milk. Combine using your hands. (Or use a stand mixer on low.) If the mixture is too wet, add a bit more oatmeal. If it's too dense, add a bit more milk. You want it to look plump, not runny.
Scoop the meat mixture into the loaf pan and smooth the top with the back of a wood spoon.
Bake at 350°F / 177°C for one hour, or until a meat thermometer reaches 150 F. Check at 30 minutes and pour off any grease before returning the loaf to the oven.
Meanwhile, combine sauce ingredients in a small pan over medium-low heat. Stir constantly until the sugar is dissolved. Remove from heat.
Spoon the sauce onto the meatloaf once it's reached an internal temperature of 150°F / 65°C. Reserve unused sauce. Return the meatloaf to the oven and continue to cook another 20 minutes (or until a meat thermometer reaches 160°F / 72°C).
Remove the meatloaf from the oven and allow it to rest 5 minutes before slicing. Top with reserved sauce as desired.