In a tupperware the same size as the block of tempeh, add the water, soy sauce, apple cider, olive oil, garlic powder, coriander, dijon mustard, and smoked paprika. Mix well and set aside.
Slice the tempeh in half (see photos). Then slice it again as many times as possible without crumbling it. Place the slices in the tupperware making sure all the slices are fully submerged in the marinade (add some more water and soy sauce until they are covered if your tupperware is too big). Allow it to marinate for 30 minutes or overnight for more flavor.
When ready to cook, heat a large pan over medium high heat. Add the oil and pick up one slice of tempeh at a time and allow it to drip the excess marinade before putting on the hot pan using tongs. Repeat the process with all of the slices of tempeh until they are all on the hot pan. Reserve the leftover marinade in the tupperware.
Cook each slice for 2 minutes (depending how hot your pan is) or until golden. Flip them and cook for another 2 minutes or until golden. Lower the heat to the lowest and pour the marinade over the tempeh. If working in batches because your pan is small, make sure to dive the marinade for the batches. Turn the heat up, place the vegan cheese on top of two slices of tempeh, cover with a lid, and cook until all of the sauce has evaporated.
Remove the tempeh from the pan and place them on a plate to cool down.
In a small bowl, mix the mayo, ketchup, sweet pickle relish, lemon juice, and sriracha or hot sauce if using. Set aside.
Lightly clean the pan where you cooked the tempeh and heat it up over medium heat. Evenly spread the vegan butter on both sides of each slice of bread and toast them on the pan until nice and toasted.
To assemble the reuben. Spread 1 tablespoon of the sauce on both slices of bread, evenly layer all of the tempeh, placing the sides with the vegan cheese as the top layer, top with sauerkraut, place the other slice of bread and enjoy immediately. (It will get very soggy if left for too long.)
Notes
Marinate the tofu/tempeh: 30 minutes is minimum, but overnight or up to 24 hours (in the fridge) is best for the most flavor.
To adjust the bread texture: Don’t overly toast it. Otherwise, it can be hard (and slightly painful) to eat the sandwich. Toasting the slices with vegan butter helps to keep them a little softer, or single-sided toasting vs. full toasting.
Leave the sauce to meld: (You can even make it 4-5 days in advance) This will help the flavors come together for improved overall flavor.
To avoid a soggy sandwich: Drain the sauerkraut well before using it.