Add the pumpkin seeds to a small food processor alongside the nutritional yeast and olive oil and pulse until the pumpkin seeds are roughly broken down.
Add the garlic, basil, spinach and lemon to the food processor and process until you get a smooth pesto.
Finish the pesto by mixing in the onion powder, salt and pepper last and either instantly serve the pesto or keep it in an airtight container in your fridge for up to 3 days.