I've tried seitan reubens, but tofu is my protein of choice in this scrumptious sandwich. Don't know why, it just is (though check this Everyday Dish video on making seitan corned beef, which may, sometime in the future, cause me to change my mind).
For now, I took slices of relatively soft, fresh tofu--though the firm kind would also work--dipped them in a little soy sauce, laid them on a plate, and microwaved them for six minutes, turning them over after four minutes. Don't let them overcook; you don't want them crunchy, just firm and a little golden on the outside.
Meanwhile, make a little sauce (I used 2 tbsp Veganaise, 1 tbsp relish, and 1 tbsp ketchup a la Robin Robertson). Take 2 slices of your bread of choice (whole wheat is what you see here), butter the outsides with Earth Balance or brush with olive oil, spread the insides generously with the Veganaise-relish mixture, then layer on the tofu, some drained sauerkraut, and some grated vegan cheese. I used cashew cheese made with agar powder instead of the sticks or threads I usually use, which came out all soft-textured and creamy, surprising me, and actually melting in the sandwich. I'll be experimenting more with that...
Fry the sandwich in a cast iron pan, or pop it into a panini press if you're lucky enough to own one, until the outside is crispy and golden and the inside is hot.
Served here with dill pickles and a new kind of coleslaw I didn't really like, so as you can see I've cropped most of it out of the picture in vengeance...but the sandwich, yes, the sandwich was just fine.