1. Heat oil in a deep pan over a medium heat. Add the onion and cook about 5 minutes until soft.
2. Add the rosemary and mushrooms and cook until mushrooms have browned (about 10 minutes). Add the wine and cook until the liquid is mostly evaporated.
3. Add the vegetable broth and simmer until reduced by one-third (another 10-15 minutes.)
4. Combine the flour, nutritional yeast, tamari, and a little of the vegetable broth from the pan in a small bowl, stirring until smooth. Add to pan and continue cooking over a reduced heat until gravy is thickened.
5. Remove from heat and add chunks of the vegan butter.
6. Season with salt and freshly ground pepper to taste.
Can be made in advance and refrigerated. Simmer when reheating.