In a large skillet, heat 1 Tbl. of the oil over medium heat. Add the onion, cover, and cook until softened, about 5 minutes. Uncover and cook until golden brown, about 5 minutes more. Scoop the onions out of the skillet and set aside.
Return the same skillet to the stove over medium heat. Add the remaining 1 Tbl. of oil and heat until hot. Sprinkle the seitan slices with the Cajun seasoning to taste, and add to the skillet. Cook until browned on both sides, turning once. Return the onions to the skillet and cook until hot, sprinkling on additional Cajun seasoning, if desired, and salt and pepper to taste. Set aside.
If using a baguette, cut it into 4 equal pieces. To assemble the sandwiches, slice the baguette portions or rolls and slather Creole mustard on the inside top and bottom. Spread some lettuce onto the bottom of each sandwich, followed by the tomato slices. Top with the seitan slices and a few slices of jalapeno, if using. Serve at once.
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