Add the soy sauce, mustard and veggie broth to a bowl, stir until combined, reserve
Mix the water and cornastach into a small bowl, reserve.
In a large skillet or wok, add the oil, then add the seitan.Stir for 1 minute on a high heat. Remove the seitan, add the garlic, stir until fragrant (40 seconds), add the vegetables. Stir for 2 minutes.
Add the sitan back to the wok, add the sauce and stir for 30 seconds.
Add the cornstarch slurry and stir some more.
Add half of the green onions, turn off the heat and serve it over noodles or rice.