Mix the soy sauce, sugar and rice wine in a large shallow bowl, then add the sliced seitan to marinate for at least an hour.
In a large frying pan, heat 1 T sesame oil over medium heat, then add the garlic, frying until you can really smell it. Then add the seitan and let it get all warm and crispy, flipping with tongs so both sides crisp evenly without burning. (Save the marinade!) After about 5 minutes, pull the garlicky seitan from the pan and set aside.
Put noodles on to cook, following package instructions. When they’re done, rinse in cold water and set aside.
In the same pan, water saute the onions and carrots for 5-6 minutes, adding more water as needed. Add the bell peppers and mushrooms, cover and reduce heat to low for about 10 minutes (adding the spinach after 5). Turn the heat back up to medium and add the seitan, noodles, cucumbers, leftover marinade and 1/4 cup water. Stir until everything is mixed and heated, then serve immediately (sprinkled with sesame seeds!).