Heat the olive oil in a large stockpot or Dutch oven over medium-high heat.
Once shimmering, add the carrots, onion, potato, celery, garlic, and mushrooms.
Let the vegetables cook undisturbed for about 5 minutes to get a good sear.
Stir once or twice and continue to cook until they are nicely browned and fragrant.
Add the parsley stems, 6 cups (1440 ml) of water, soy sauce, and a few grinds of black pepper to the pot.
Stir everything together, scraping up any browned bits from the bottom of the pot.
Bring the mixture to a rolling boil, then immediately reduce the heat to low.
Let the stock simmer gently, partially covered, for at least 30 minutes. The longer it simmers, the richer the flavor will be, so feel free to let it go for up to an hour.
Carefully strain the stock through a fine-mesh sieve into a large bowl or container, pressing gently on the vegetables to extract all the liquid.
Discard the solids.
Taste the finished stock and adjust the seasoning.
Add a pinch of salt or another splash of soy sauce if needed. Use immediately or let it cool completely before storing.
Don't rush the browning process in the first step. Those caramelized bits on the bottom of the pan are pure flavor, so let the veggies get some real color.
Hold off on adding salt until the very end. As the stock simmers and reduces, the flavors concentrate, so you'll have better control over the final seasoning if you wait.
