Heat the olive oil in a large skillet over medium heat and saute the onion until it starts to soften, about 5 minutes.
Add in the garlic and mushrooms and stir for 1 or 2 minutes, just until the garlic is fragrant. Add in the potato, tamari, and water and bring the water to a boil.
Once boiling, lower the heat to a simmer and cover the pan. Cook until the potatoes are fork-tender, about 10 minutes. (The smaller you cut the potatoes, the faster they will cook.)
Once tender, carefully transfer the contents of the pan to a high-speed blender or food processor, and add in the salt and black pepper. Blend until smooth, using a thin towel to carefully cover the vent at the top of your machine so that the steam can escape. (And not blow the lid off your blender!)
Adjust any seasoning to taste and serve warm over your favorite side dish or main course. Leftover gravy can be stored in an airtight container for up to 3-4 days in the fridge. It will thicken in the fridge, but will thin-out when reheated. You can always add an extra splash of water to thin it out even more, if needed, but don't add the water until the gravy is warm first.
Nutrition information is for roughly 1/4 cup of gravy. This information is automatically calculated, and is just an estimate, not a guarantee.If you don't find Yukon gold potatoes at the store, red potatoes would be the most similar substitute. Or, you could try using parsnips or turnips, too.