Pick the basil leaves onto a chopping board (reserving a few baby leaves to garnish), then roughly chop the remaining leaves and finely chop the stalks.
Peel and finely slice the onion and garlic.
Cut the tomatoes in half, and then roughly chop.
Heat oil in a saucepan on a medium heat and add the onion, then cook for around 7 minutes, or until soft and lightly golden.
Stir in the garlic and basil stalks for a few minutes, and then add the tomatoes and vinegar.
Season with a tiny pinch of salt and pepper. Continue cooking for around 15 minutes, stirring occasionally.
Stir in the chopped basil leaves, then reduce to low and leave to simmer.
Meanwhile, fill a large pot three-quarters of the way up with boiling water, add a tiny pinch of salt and bring back to the boil. Add the spaghetti and cook till al dente.
Once the pasta is done, ladle out and reserve a cup of the cooking water and keep it to one side, then drain in a colander over the sink and tip the spaghetti back into the pot.
Stir the spaghetti into the sauce, adding a splash of the pasta water to loosen, if needed.
Serve with the reserved basil leaves sprinkled over the top and sprinkle with grated parmesan.