Cook the onions in a skillet over medium heat until they’re soft.
Add a bit of water to the skillet if they begin to stick.
Add the mushrooms and continue cooking until they’re cooked well.
Remove the pan from heat and add the flour, nutritional yeast, sage and dry mustard.
Stir until all the ingredients are combined well.
It will look pretty dry at this point.
Dissolve the miso in the water and add the tamari.
Return the pan to medium-low heat and stir in the miso water.
Cook while stirring until the gravy thickens, about 5 minutes.
This will thicken more as it cools, so add more water if it’s thicker than you like.
If you have leftover gravy it may be too thick.
If so you can add water when you reheat it.
Notes
Note: You can skip the miso in this, but it does give a great flavor. We'll use it for other recipes too and it lasts a very long time in the fridge - like 3-12 months.