Melt the butter in a large deep frying pan. Add the courgettes and onion. Stir and sauté for about 4 minutes until the onion is starting to soften. Cover the pan, and sauté for a further 3 minutes until soft and translucent. Stir in the cream, and allow it to come to a gentle simmer.
Remove the pan from the heat, and stir in the Parmesan and nutmeg. Season with salt and pepper. Keep warm.
Bring a large saucepan of lightly salted water to the boil, and add the fresh fettuccine. Return to the boil, then cook for 15 seconds. Drain andadd to the frying pan with the sauce. Toss everything together, and serve immediately