Put the rice in a medium saucepan. Cook according to package directions. Lower the heat so it bubbles gently but steadily; cover and cook, undisturbed, until the rice is tender and the water is absorbed, 25 to 30 minutes (check only toward the end of cooking time). The rice should be studded with craters, or steam holes when it is ready. Remove the pan from the heat and keep it covered.
Meanwhile, place the 4 teaspoons of tamari soy sauce and 1/4 cup of ice water in a bowl and stir to combine. Put the 1 pound seitan strips in the bowl and marinate for 10 minutes; set aside. No need to refrigerate.
Rinse and dry the snow peas, napa cabbage, and scallions. Cut the peas into strips; core and chop the napa cabbage. Trim and chop the scallions, keeping the whites and greens separate. Trim, peel, and mince the ginger and 3 garlic cloves.
In a small bowl, put the 1/4 cup of tamari soy sauce along with the 1/4 cup of brown or coconut sugar and 4 teaspoons of cornstarch, then add 1/2 cup of water and stir to combine and whisk; set aside.
Drain the seitan strips and reserve the liquid. Toss the marinated seitan strips to 1 1/2 tablespoons of cornstarch
Put 1 tablespoon of vegetable oil in a large skillet over medium-high heat. When it’s hot, add and stirfry the seitan, adjusting the heat to prevent burning, until the bottoms are crisp, stirring occasionally until the seitan is browned on all sides, 5 minutes. Remove and keep warm
Add the remaining 1 tablespoon of vegetable oil to the pan. When it’s hot, add and stirfry the garlic cloves, minced ginger and chili peppers, and cook until they’re fragrant, about a minute. Add snow peas, stirfry for 1-2 minutes; then add scallions and napa cabbage, 2-3 minutes.
Add the tamari soy sauce mixture, ginger, and peas and toss to coat in the sauce. Continue cooking until bubbling and thickened. Return the seitan to the pan and toss gently to coat with the sauce.
Serve the stir-fry atop the rice and sprinkle with sesame seeds.