Heat the oil over medium heat in a skillet until hot. Add in the onions, garlic and mushrooms, stirring around for a few minutes until the onions are translucent.
Add in the broth, Italian seasoning, tamari and a little bit of salt and pepper. Stir for a minute, then whisk in the cornstarch to avoid clumps.
Reduce heat to low and simmer until thickened, stirring every few minutes, for about 20 minutes. It should be a rich brown color.
Once it's done, taste test the mixture and add more salt, pepper or seasonings as desired.
Remove from the heat and add the gravy to a serving bowl. Enjoy over top of mashed potatoes or your dish of choice!
Notes
Storage
Store in an airtight container in the refrigerator for 3 to 4 days.