Heat oil in a large sauté pan. Add mushrooms and onions and sauté over medium-high heat until brown, about 10 minutes, stirring occasionally.
Add about 3/4 of the vegetable stock as well as all of the garlic and thyme. Bring to a simmer and cook for an additional 6-8 minutes.
While that simmers, whisk the remaining stock with the wine, tamari, yeast, cornstarch, and black pepper in a small bowl. Gradually add to the simmering sauce to allow the gravy to thicken considerably. Cook for an additional 2 minutes.
Prior to serving, add the Tomato Secret and simmer for 5 minutes.