LOL. Had an In-N-Out burger for the first time last year. *Does.not.get.the.hype.* When you can already make a burger 1000 times better than that by default, why copy that one?

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Mysteries...

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On a helpful note, though... You can do it with a cast iron. Flat iron works good, also, but there are different things that can classify as such. For example, I have a flat iron that I made on a CNC plasma cutter. I cut what looks like a 3/8" thick mild steel manhole cover, with handles on it. I use it on my gas grill, and cook everything from burgers to indian flat breads. (roti, paratha, naan, etc)Â
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Aside from the joking - I really don't get the appeal of the In-N-Out - but that cast iron will serve you well for possibly one of the best burgers that one can make. That is, the smashburger.  Seasoned meat, with some onion, garlic, and hot peppers rolled in. Just put a ball of that seasoned meat in a hot, buttered cast iron, and squish it with a spatula. Boom.