Melt the butter in a saucepan, sauté the onions and garlic, add the spinach and pour in water. Steam until soft with the pot closed. Put the cream and processed cheese in the pan with the spinach, dust the flour over it, puree finely with the puree stick, bring everything to the boil briefly and season with salt, pepper and nutmeg. Pour over the cooked spaghetti and serve.