I make seitan deli meat loaves in two flavors: Turkey and ham. This is the ham variety. Sliced thin, and lightly toasted in a pan with a dash of oil. Let the edges get crispy, pile it up, and slap a piece of fake cheese on it. Cover the pan and let melt.
In a different pan, sauté some diced onion in olive oil until slightly carmelized. Turn off heat, add horseradish mustard, mix.
Toast ciabatta bun, spread the onion/mustard mix on, top with the meat/cheese pile, load up some pickles and go crazy.
I have a pic of the inside after I took a bite which better shows the texture of the “ham”, but I have no idea how to add it.
A while ago there used to be a poster who went by potato who was the seitan master and even owned a whole deli slicer to make sandwiches with lol! I can’t remember their recipe now but I found their recipe box https://www.copymethat.com/recipebox/im-vegan/4507910/