I make these quite often. Two comments tho.
1- there needs to be more onions than what's used in the video, IMHO
2- try slicing real thin, I use a mandolin, put in a strainer with a tsp of so of kosher salt, mix good then let sit for 10-15 minutes and squeeze them really good.
I'll try a little onion to test for saltiness then adjust the amount I salt the burgers accordingly. Sometimes I will put on the grill then smash the burger on it, other times I will put them on after flipping.
Either way, these are some fantastic burgers.