In a large skillet or frying pan, melt the butter over medium heat. When hot, add the onion, red bell pepper, and garlic. Sauté for 3 - 5 minutes until the vegetables soften. Push the veggies to the side of the pan and add the seitan steak slices, adding more butter if needed. Fry the steak slices for about 5 minutes, stirring as needed until it's browned. Mix the steak and onions mixture together, turn off the heat, and mix in the leftover steak marinade.
Prepare the sandwiches by dividing the steak and onion mixture onto the two buns and dollop the nacho cheese on top. Serve hot.
This recipe is enough to make 2 sandwiches, but feel free to double or triple it to make more!These sandwiches are quick to make (about 15 minutes) assuming you have already prepared the steaks and cheese ahead of time. If you are preparing everything fresh, the seitan steaks take about 45 minutes and the nacho cheese takes about 15 minutes.