>>21517011 (OP)
I only know the Krystal recipe.
Mustard, onions, steamed buns.
It is also how they cook the burger "animal style" in mustard and onions at In n Out burger.
I occasionally have tried to duplicate it at home. I think getting the meat paper thin is best acheived with baby meatballs and the smashburger style. Use something heavy to weight down your spatula with the press. Holes are not needed, just smother it all with minced onions. I use vidalia sweet onions. I use lawrys and pepper, but that's me.
