Marinate the Seitan: In a bowl, combine the seitan (or tofu/tempeh), soy sauce, olive oil, brown sugar, maple syrup, and apple cider vinegar. Mix well and set aside to marinate for 20 minutes.
Sauté the Vegetables: Heat the sesame oil in a large skillet or wok over medium heat. Add the chopped yellow onion and sauté for about 5 minutes, until fragrant and slightly softened. Add the broccoli florets and 1 tablespoon of water. Cook for 6-8 minutes, until the broccoli is tender-crisp.
Prepare the Sauce: In a small bowl, combine the sesame oil, rice vinegar, minced garlic, minced ginger, maple syrup, chili garlic sauce, soy sauce, brown sugar, and water. In a separate small bowl, mix 2 tablespoons of the prepared sauce with the cornstarch to form a slurry. Set the slurry aside.
Combine and Cook: Add the pre-cooked udon noodles to the skillet with the onion and broccoli. Cook for about 2 minutes, then add half of the sauce and toss until the noodles and vegetables are evenly coated. Add the marinated seitan (or tofu/tempeh), the remaining sauce, and the cornstarch slurry. Stir to combine and cook for another 3-4 minutes, until the sauce thickens.
Serve: Serve the noodles immediately, topped with your favorite garnishes like sesame seeds, sliced green onions, and chopped cilantro.
Storage and Freezing: Store leftovers in an airtight container in the fridge for up to 4 days, or freeze for up to 2 months. To reheat, warm in a skillet, adding a splash of water or broth if needed. For best texture, consider freezing the seitan and vegetables separately from the noodles and cooking fresh noodles when ready to serve.