Gluten-Free & Vegan Gravy

olive oil ((or water))

yellow onion (, chopped)

garlic cloves (, minced)

mushrooms (, sliced (about 1 cup loosely packed))

medium Yukon gold potato (, cut into 1-inch chunks (4-6 ounces by weight))

tamari ((gluten-free soy sauce))

water

fine sea salt

black pepper