Heat oil in a medium saucepan over medium heat. Saute onion and garlic until soft and translucent, about 5 minutes. Stir in flour, nutritional yeast, and soy sauce to form a smooth paste. Gradually whisk in the broth. Season with sage, salt, and pepper. Bring to a boil. Reduce heat, and simmer, stirring constantly, for 8 to 10 minutes, or until thickened.
