In a pan on medium heat saute your onions and mushrooms in a little veggie broth or olive oil for 2 minutes.
Add in your spices and stir to incorporate, cook another 2 minutes.
Add in the coconut aminos and saute another 2 minutes.
Add in your oat milk and let simmer gently for 5 minutes.
Make a slurry by stirring together 1 tbsp tapioca flour and 2 tbsp broth or water in a small bowl.
Add the slurry to the gravy and stir constantly for 1 minute then remove from the heat, it will continue to thicken. Serve warm over mashed potatoes.