Bring a high-sided nonstick skillet to medium heat on the stovetop. Add in just a small splash of your vegetable broth (no more than 1-2 Tbsp) and your onion and garlic. Allow to sauté for 5 minutes, adding more vegetable broth as needed to prevent sticking (only adding a splash at a time).

Add your mushrooms and tamari to the skillet and allow to cook for 6-8 minutes. Stir occasionally and continue to add splashes of vegetable broth as necessary to prevent sticking (although the mushrooms should start releasing their own juices after a few minutes, so you shouldn't need very much!).

Add in your flour and stir for one minute to heat. The flour should start to cling to the mushrooms and onions as you do this - if the mixture is too dry and this is not happening, add in another 1-2 tablespoon of vegetable broth.

Add in the rest of your veggie broth (you should still have close to 3 cups, minus just a few tablespoons!), and your thyme and sage. Stirring regularly, bring this mixture to a simmer (this took about 8 minutes for me). If you notice that your flour is trying to clump up at all, feel free to use a whisk (one that is safe for non-stick cookware!) to get it as smooth as possible.

Add your nutritional yeast, if using. Stir to incorporate. If it has already reached your desired consistency, move onto step 7. If you want it thicker, allow to cook another 5-8 minutes until it reaches desired consistency. Note that it will get slightly thicker as it cools.

Add salt to taste and remove from heat. Serve as desired and enjoy!