Cook the spaghetti according to the instructions on the packet. Drain and set aside.
Heat the oil in a saucepan. When hot, gently fry the onion until soft and translucent. Add the chilli and fry for a further minute before adding the garlic.
Increase the heat and add the mussels (first discarding any that don't shut when tapped), clams and wine. Cover the pan and steam the shellfish until they open, discarding any that don't. Strain through a sieve, reserving the liquid in a bowl.
Return the liquid to the pan, leaving a small amount in the bowl, as this may contain some grit from the shellfish. Boil the liquid for 2 minutes until it has reduced slightly. Whisk in the butter, then stir in the shellfish, spaghetti and parsley. Season well and drizzle with a little olive oil.