Put roughly half the spinach into the food processor, then add walnuts, nutritional yeast, lemon juice, water, and salt. (If you have a large food processor, you can add all the spinach at once.) Process on high, pouring in olive oil as the blade spins. Scrape down the processor’s sides with a spatula as needed. Add remaining spinach and poke it down a bit into the pesto, then process until almost smooth. Taste, then add a few grinds of pepper or adjust other seasonings if desired, processing to combine.