Reduce heat to medium-low, and add a splash of oil if skillet is dry. Cook garlic, ginger and red pepper flakes until fragrant, 1 minute. Stir the stir-fry sauce and add sauce and scallions to skillet. Cook until sauce is thickened, about 2 minutes. If pan looks dry, add some of the reserved liquid from step 2. Toss in seitan and stir to coat with sauce. Serve with cooked rice, and garnish with sesame seeds and additional sliced scallions.