Bring a large pot filled with about 4 quarts water and plenty of salt (about 2 tbsp) to boil over high heat. Add the pasta and cook according to the package instructions. Add the spinach once the pasta is done and stir it in to wilt down. Drain and set aside (or transfer back to the pot if your sauce is ready!).
While the pasta cooks, in a high-speed blender, add your pumpkin seeds, nutritional yeast, and salt and blend on high into a fine parmesan-like texture with some chunks. Transfer to a bowl.
No need to clean the blender. Add parsley (if using), avocado, water, olive oil, lemon, miso, garlic, and salt. Blend until smooth. If it is not blending, add more water, 1 tablespoon at a time, until it blends easily. The amount of water you’ll need depends on the size of your avocado since they vary so much. The texture should be thick and it will be a bit gloppy from the avocado. Taste and add more lemon or salt, if needed. You should have about 2 cups of sauce.
Add pasta back to the pot, pour the avocado sauce over top, and stir to coat. Serve topped with pumpkin seed parm and lots of black pepper. Only sauce what you’ll eat - leftovers should be stored separately.