We’ve stuffed a meat loaf with shredded cheese, then cooked it over a rich onion mixture that tastes like French onion soup. Wow! Brown those onions well. Stop short of burning them black, of course; but they must be deeply colored — a rich, dark brown.
One note: Because you have to pick up and remove the hot meat loaf (on its rack) from the pot to then thicken the sauce, it’s best
to use a rack with arms that can stand up in the insert— or even a silicone rack with flexible, silicone arms (so long as it doesn’t have wide gaps between the slats).