Heat the olive oil in a large saucepan over a low heat, add the onion and garlic and cook for 12–15 minutes or until softened. Add the chilli, if using, tomatoes, white wine and parsley and season with salt and pepper. Bring to the boil, then reduce the heat and simmer for 2–3 minutes.
Add the clams, cover with a tight-fitting lid and cook gently for 4–5 minutes or until they have opened. Discard any clams that remain closed.
Meanwhile, cook the spaghetti in a large saucepan of salted boiling water for 10–12 minutes, or according to the packet instructions, until al dente. Drain well and place in a serving dish.
Heap the clam and tomato sauce over the spaghetti, drizzle with the truffle oil and sprinkle with pepper. Serve immediately with crusty bread.