Make the sauce: combine the ketchup, soy sauce, water, honey, cumin and cayenne pepper in a bowl, and set aside.
Heat the oil in a skillet on medium high. Add the onions, cook for a few minutes, stirring occasionally.
Stir the peppers into the skillet, cook for about 5 minutes, stirring occasionally. The peppers and onions should be starting to brown.
Add the seitan slices, then stir in the sauce. Cook for about 3 more minutes, so the sauce gets combined with the seitan and veggies, and the flavors meld.
Place a tortilla on a plate, and scoop in a portion of the seitan mixture. You can add toppings like sour cream, guacamole or cheese if you like; but it’s pretty tasty on it’s own. Enjoy!