Preheat a 12-quart stock pot over medium-high heat. Sauté the onion, garlic, and leeks in oil with a pinch of salt for about 5 minutes, just to get a little caramelization going.

Add the carrots, celery, parsley, dill, thyme, rosemary, peppercorns, and nutritional yeast and give a stir. Add the water. Cover pot and bring to a boil. Once boiling, leave the lid slightly ajar so that steam can escape, and lower heat a bit, simmering for 1 1/2 hours, to reduce by about half. Stir occasionally.

Turn off heat and let cool in the pot, just until it's not steaming. Now is the time to taste for salt. Start with a teaspoon and go from there, adding pinches until it's just right.

Place a separate large pot in the sink. It should be big enough to manage 4 quarts of broth. Position a handled strainer in the empty pot and make sure it's secure. Line the strainer with a few layers of cheesecloth with plenty of overhang.

Pour in the vegetables and broth and leave them in the strainer to drain for about 20 minutes.

Once cool enough to handle, bunch up the cheesecloth and squeeze, so that the vegetables release as much moisture and flavor as possible.

Taste for salt again, and now your broth is ready to use! Freeze in tightly sealed containers for up to 3 months, if not using within the week.