Add in the onions and cook a few minutes. Add in the mushrooms and seitan and cook until cooked through and liquid has evaporated. Sprinkle with salt and pepper. Set aside.
In a separate pan, heat up some butter.
Butter Slices of bread and make sandwiches, piling cheese then filling then more cheese.
Grill in pan until brown on each side and cheese has melted.
Serve.
Enjoy!
Recipe by Confident Cook, Hesitant Baker at https://confident-cook.com/2016/04/italian-sausage-monterey-jack-grilled-cheese.html