When the second burger is able to flip, drizzle with mustard as earlier than, flip and prime with cheese. Sit the primary burger on prime of it to make a double stack, then prepare dinner for two minutes, or till the onions beneath are softened and charred. Switch the burger stack to the tray within the oven to maintain heat. Repeat with the remaining beef, onions, mustard and cheese to make 4 burger stacks.
