Create a roux by combining flour and 3 tablespoons oil in a small bowl and whisking well.
Place remaining tablespoon of oil in a sauce pan on medium high heat. Add onion, garlic and mushrooms, and cook until onions are translucent, approximately 5 minutes, stirring frequently.
Add water, nutritional yeast, soy sauce, sage and bring to a boil, stirring frequently. Reduce heat to simmer, add roux and stir constantly until sauce thickens. Add parsley, salt and pepper to taste and stir well before serving.