In a medium saucepan combine the first six ingredients (through mustard) and ¾ cup water. Bring to boiling; reduce heat. Cover and simmer 30 minutes. Add nutritional yeast. Use an immersion blender to blend until smooth.
Heat sauce over medium 3 to 4 minutes or until thickened, stirring constantly. Stir in cornstarch and vinegar; season with salt and pepper. Store in an airtight container in the refrigerator up to 3 days.