Heat oil in a pan over medium heat and add peppers and onion and cook for a few minutes until they start to soften and onions begin to carmelize.

Add the cheese and lightly mix until it melts. Transfer veggies to a plate.

Add the sliced seitan to the pan and cook for a few minutes on each side, until it is warmed through and the edges are browned and crisp.

When the seitan is done, add the veggies back in and mix until well-combined.

Add the filling to the bun and serve!