Add some wood chunks for smoking if you like. I use 2-3 cherry wood chunks, but apple wood, oak wood or beech wood works just fine.
Place the meat on the indirect side and close the lid. Wait until the inner temperature reaches 45° C / 113° F. It takes roughly 30 minutes.
In the meantime, slice the onions and chop the mushrooms and fry them in a frying pan using either beef tallow or regular butter, until soft.
Step 3 - sear
Now stoke the fire to get it real hot. Normally I sear over hot coals, but with so much fat on this steak it's too easy to have a grease fire, so in this case I use a cast iron pan. Place it on the grates over hot coals.
Now sear the steak in the pan, without adding any oil or butter. Leave it for 1-2 minutes, and then flip it, and sear it for another minute. Check the meat for done-ness, its internal temp should reach 55° C / 131° F.
Now build the sandwiches, applying (from bottom and up): horseradish, onions, spinach leaves, beef and then mushrooms on top.