This recipe is courtesy of Gusta.

1. Preheat oven to 180°C (350°F).
2. In a saucepan, melt 60 mL (¼ cup) of margarine over low heat. Add flour and cook for 1 minute while stirring. Slowly whisk in vegetable broth until no longer lumpy.
3. In a bowl, dissolve miso in hot water. Add miso liquid, soy sauce, onion powder, and pepper to broth and bring to a boil. Reduce heat to medium and whisk until sauce thickens. Set aside.
4. Melt 5 mL (1 tsp.) of margarine in a skillet set over medium heat. Add mushrooms and cook until golden, about 5 to 7 minutes. Stir in reserved sauce. 
5. Cut baguette into two 15-cm (6-in) long pieces and cut in half to open.
6. Thinly slice seitan loaf. In a skillet set over medium heat, add coconut oil and brown both sides of each seitan slice.
7. Place roasted seitan slices in each baguette and top with grated cheese. Keeping sandwich open, place on a baking sheet and bake for about 10 to 15 minutes, or until cheese melts. Remove from oven and pour sauce over sandwiches. Enjoy!