Heat two-thirds of the butter in a small pan over low heat and sauté the spinach for about a minute, until just wilted. Transfer the spinach and butter into the bowl of a small food processor or blender and process to a smooth purée.
Cook the pasta in plenty of boiling salted water according to the packet directions. In a large frying pan, heat the remaining butter over medium heat and fry the anchovies (if using), garlic and onion until fragrant.
Add the zucchini and salt and toss for about 3 minutes until softened.
When the pasta is cooked to your liking, transfer it into the zucchini and toss to combine. Remove from the heat and stir through the spinach puree.
Serve topped with a little parmesan cheese.
*Omit the anchovies for a vegetarian version
