Add 1 tablespoon olive oil to a hot pan. Add chopped onion and cook on low heat for about 35 minutes, or until onions are caramelized. Add a little water as needed (about ¼ cup) at a time as needed.
Add balsamic vinegar to deglaze the pan. Add salt.
Add sliced mushrooms and the rest of olive oil. Cook about 5 minutes, or until tender. Add salt and pepper.
Add gluten free flour. Cook 2 minutes. Add vegetable broth and tamari. Bring to a boil. Reduce heat and simmer about 5 minutes to thicken.
Take ½ the cooked mushrooms and onions and blend in a food processor. Add back to gravy.
Notes
This recipe will last 5 days in an airtight container in the fridge.