It's more how they cook it and prep the patty. And it doesn't sound nearly as good as it tastes.
Form the burger fairly thinly. Then you poke holes in it, pencil sized holes, works best with the patty partially frozen. If the patty is thawed, the holes make the patty fragile, You can move it carefully in the raw state, but a partially frozen patty holds together better.
Chop onions fine. Make patty sized mounds of onion in the pan (this is a time I like non-stick or cast iron) Salt and pepper the onion. Lay the patty on the onions and season it. Cover with a lid and let the patty cook in the onion steam.
Turn it once and cook the other side on top of the onion pile. You may have to reform the onion pile before putting the burger back down on it. When done, scoop the whole pile onto your prepped bun.
The juices and sugars from the onions and meat make a caramely mess on the pan that is a pain to clean but tastes gooood. That's why I like non-stick or well seasoned cast iron for this recipe.
Could be fun to try shallot one of these days.
more than taste fine
me eat it all the time