Place the oats, water, and salt in a saucepan. Bring to a boil, reduce heat to low, and simmer until the oats are starting to thicken but aren't yet fully cooked (about 3-5 minutes). Stir in the nutritional yeast, lemon zest and juice, and then add the spinach; cover the oats and allow the spinach to wilt down for about a minute. Then, remove the lid from the oats and stir the spinach in. Continue cooking for another 2-3 minutes, or until the oats are creamy and have reached a desired consistency. If they get overly thick, you can always add a splash of water. Taste and adjust salt and pepper as needed.