Heat the oil in a saucepan. Add the onion and fry over a low heat for 3–5 minutes, until soft. Stir in the carrots, leek and celery, then the tomatoes, tomato purée, cayenne pepper and mushrooms. Add a pinch of salt and pepper and simmer for 10 minutes.
Meanwhile, cook the spaghetti in a saucepan of lightly salted boiling water for 8–10 minutes, or according to the packet instructions, until al dente. Drain the pasta and sprinkle with pepper. To serve, mound up the spaghetti and spoon the sauce over the top. Garnish with basil leaves.