Heat the oil over medium heat, add the onion, and sauté for 3-4 minutes, until starting to turn translucent.
Slowly add the flour, stirring constantly. Cook for a couple minutes, until yellow.
Continue whisking as you pour in the seitan broth and sage. Simmer, whisking frequently, until thickened.
Serve with mashed potatoes or as part of wheat meat and potatoes.