Next-level smash burger recipe | Good Food

When the second burger is ready to flip, drizzle with mustard as before, flip and top with cheese. Sit the first burger on top of it to make a double stack, then cook for 2 mins, or until the onions underneath are softened and charred. Transfer the burger stack to the tray in the oven to keep warm. Repeat with the remaining beef, onions, mustard and cheese to make four burger stacks.