In a small bowl, prepare your sauce by combining sesame paste, black vinegar, oliosaccharide, toasted sesame oil, and soy sauce and stir well until combined.
Bring a pot of water to boil, then add your udon noodles and cook according to package instructions. Drain but reserve about 2-3 tbsp of cooking water for the sauce.
Meanwhile, if making seitan, pan fry the seitan. You can add some oil for extra crispiness.
Add the prepared sauce to your drained noodles with the 2-3 tbsp of cooking water, and stir well until the noodles are coated.
Divide into two plates, top with seitan, sliced cucumber, green onion curls, and toasted sesame seeds. Eat immediately.