Bring a pot of water to a boil and add tomatillos. Cook for about 5 minutes, or until a dull olive color. Drain and place in a blender along with ½ cup chopped onion, cilantro, garlic, chiles, and salt. Process until smooth. Heat 2 tablespoons oil in a large saucepan and add puréed tomatillo mixture. Stir until mixture is boiling evenly, and then reduce heat to simmer. Cook until reduced to a medium-thick sauce, about 5 minutes. Add seitan pieces and let cool slightly.