1. Melt half of the butter in a large saucepan, add the onions and garlic and sweat for one minute.
2. Add the spinach and stir well before adding the sage leaves. Season with nutmeg and a little salt and pepper.
3. Cook until the spinach is wilted (about 1-2 minutes) before removing from the heat. Drain well to remove the water and place in a food processor and puree.
4. Meanwhile cook the spaghetti in plenty of boiling salted water until cooked but still firm. Drain, stir in the remaining butter and check the seasoning.
5. Stir in the spinach, sprinkle the Parmesan or Old Winchester over the top and serve immediately.
Recipe courtesy of Great British Chefs. For more pasta recipes visit