Heat oil in a medium saucepan over medium heat. Add onion and garlic; cook, stirring often, until softened, about 5 minutes. Add mushrooms and cook, stirring often, until they begin to release their juices, about 10 minutes.Advertisement
Add broth, tamari (or soy sauce), thyme and sage; simmer for 10 minutes. Mix cornstarch and water in a small bowl. Stir into the sauce and simmer, stirring often, until slightly thickened, about 10 minutes more. Season with pepper. If you prefer a smooth gravy, pass it through a fine sieve (discard mushrooms and onions). Serve hot.
Make Ahead Tip: Cover and refrigerate for up to 2 days. Reheat on the stovetop or in the microwave.
Commercial vegetable broth is readily available in natural-foods stores and many supermarkets. We especially like the Imagine and Pacific brands, sold in convenient aseptic packages that allow you to use small amounts and keep the rest refrigerated.
People with celiac disease or gluten-sensitivity should use soy sauces that are labeled "gluten-free," as soy sauce may contain wheat or other gluten-containing sweeteners and flavors.