To make the sauce, first stir the cold water and cornstarch together in a small bowl, until the cornstarch completely dissolves. Stir in the soy sauce and brown sugar. Set this aside for now.
Place the cornstarch into a medium bowl and add the seitan strips. Stir gently to dredge the seitan in the cornstarch.
Coat the bottom of a large skillet with the oil and place it over medium-high heat. Add the seitan in as even a layer as possible. Cook the seitan strips until they're browned and crisp on the bottom, about 4 to 5 minutes. Flip and cook them another 4 to 5 minutes more, until the strips are crispy on opposite sides. Remove the seitan from the skillet and transfer it to a plate.
Lower the heat to medium. Add the garlic, ginger, and dried chilies, if using, to the skillet. Cook everything for about 1 minute, stirring constantly, until very fragrant and the chilies begin to darken a bit.
Return the seitan to the skillet, then pour the sauce over it. Stir everything well and cook the seitan in the sauce for 1 to 2 minutes, until the sauce has thickened slightly and coats the seitan strips.
Add the scallions to the skillet and stir everything up, then remove the skillet from heat.
Divide the seitan onto plates and serve with rice or noodles.