Dissolve the arrowroot or cornstarch in a little water. Whisk to combine in a small bowl with the remaining sauce ingredients.
Heat the oil in a wide skillet or stir-fry pan. Add half of the oil and add the onion. Sauté over medium-low heat until translucent.
Add the peppers and continue to cook until the onions and peppers are lightly browned. Transfer to a plate or bowl and set aside.
Heat the remaining oil in the skillet. Add the seitan and sauté over medium-high heat, stirring frequently until golden brown on most sides. Pour in the sauce, reduce the heat to medium, and cook for a minute or so longer, until the sauce has thickened and the seitan is nicely glazed.
Stir in the onions and peppers and serve at once, atop or alongside hot cooked grains, if you'd like.