In a large pan with canola oil, sautée the chopped onion and garlic cloves until soft. Add the cumin and fennel seeds. Let it cook further until the spices are toasted.
In a food processor, add the chickpeas, onion-garlic-spices mixture, tomato paste, salt, thyme, smoked paprika, soy sauce, and sparkling water. Pulse until it’s saucy. Then add the vital wheat gluten and pulse again until it comes together in a ball.
Prepare a large pot (filled with a couple of inches of water) with a steaming basket and bring to high heat.
Divide the seitan into 8 equal chunks. Add the seitan chunks into the steaming basket and let them steam for about 40 minutes, flipping them after 20 minutes.
In the meanwhile, you can prepare the homemade cheesy sauce: Heat the coconut oil or butter in a small pot over low heat. Add the salt, turmeric powder, curry powder, and paprika powder. Add in the flour and whisk. Add nutritional yeast, whisk again until clumpy. Add the water and whisk on low heat for 1-2 minutes until it gets gooey. Set aside. If it gets too thick later on you can add a little bit more water and whisk again.
Let the seitan cool off a bit before slicing it into thin strips.
Bring a large pan with olive oil to high heat, toss in the white onion and let it cook until translucent. Then add the bell pepper and seitan strips and let it pan-fry for 3-4 minutes until the seitan is crispy on the edges and the bell pepper is softened. Add more salt to taste.
Fill the sandwiches with crispy seitan and add the cheese sauce on top. Enjoy!