Melt half of the butter in a heavy skillet and cook the onions and garlic for just a minute until softened slightly.
Add the baby spinach to the onions and garlic in the pan and give a stir before adding the chopped sage. Season with salt and pepper and a little ground nutmeg.
After a couple of minutes when the baby spinach is well wilted, remove from the heat. Drain off any excess liquid and purify the mixture in the food processor.
Bring a large pot of water to the boil, salt well and cook the spaghetti until al dente. Drain the spaghetti and toss with the remaining butter. Check the seasoning.
Stir the spinach puree into the spaghetti and serve with plenty grated parmesan.