In a sauce pan, melt 2 tablespoons of butter.
Add the onion, garlic, and mushrooms and sautee for 8-10 minutes, or until it cooks down and the mushrooms release their liquid.
Remove the mushroom mixture from the pan and place it into a blender or food processor.
Blend the mushroom mixture with 1 cup of the vegetable broth until smooth, then set aside.
Melt the remaining 6 tablespoons of butter in the sauce pan.
Add the flour and whisk constantly for 1 minute.
Add 2 cups of the vegetable broth and the tamari, and continue stirring.
Add the blended mushroom mixture and continue to stir as it thickens.
Continue whisking the vegetarian gravy for 5-10 minutes, or until the mixture thickens and coats the back of a spoon.
Add the sage, then salt and pepper to taste before serving.