It tastes like summer. That's all I can say.
The key to this is the ratios. The ratio of the beef to the smoke to the relish. The smokey to the tangy to the meaty. If you nail the ratios right, this is a fantastic sandwich. If you don't nail the ratios right, its a good sandwich.
First the relish part. Shred some cabbage, carrots and chop some green onion. Grab a jar of dill pickles and chop all the pickles. Place all the vegies in a bowl, add a little salt and then cover with the pickle brine. Then cover and place in the fridge overnight. If you want a little heat put a couple chopped jalapenos in the mix. You want to smash everything in the bowl and make sure the pickle juice just covers it. As the cabbage and such absorb the brine, you may need to stir a few times.
Take whatever chunk of beef you like. Rump, chuck, loin even brisket and you're going to smoke it. Here's one of the keys in the ratio; In the smoke scale from light (like a pellet grill) to burnt tire (like when you burn nothing but bark), you want a very nice heavy medium. The smoke flavor needs to be there. I always prefer hickory, oak or mesquite for beef and either of those woods will pair well with this.
Once the beef is done to the doneness you prefer, you're going to slice it and of course against the grain. Then we need to butter the bread and lightly toast until golden brown and plate up.
And please don't put ketchup on this. Have some humanity for our world. Mustard sauce would be good.