In a large stock pot, sauté seitan, onion, carrots, and celery in the olive oil until the seitan is lightly browned.
Add broth and potatoes and bring to a boil. Reduce to simmer and add sage, rosemary, and thyme. Cover and cook for 30 minutes or until potatoes are soft.
In a small bowl, combine cornstarch and water. Whisk until a smooth consistency. Slowly add to soup, stirring well to avoid lumps.
Continue to cook uncovered for an additional 5 minutes or until stew has thickened.
Season with salt and pepper to taste.