Some stores steam the paddies with minced onions rather than frying them. The bun is also placed under the steamer and absorbs some of the meat flavor.
The first hamburger stand of this type I encountered was Milligan’s BeefyBurger in Orlando. The “Beefy” was a confession that it was not all beef but some oatmeal and soy flour. They sold for 12 cents each with mustard and dill pickle and were worth every penny.
It’s the steaming with the onions. They deal them out like cards along the griddle and then layer the buns on top after dipping onions along them.
I can’t stand this. I’m getting hungry for them just thinking about it. I live in FL now, I only get bellybombers when I go back up north.