Remove the stem from the tomatoes and make a cross incision on the bottom. Immerse them for a few seconds in boiling water, then place in iced water to stop the cooking process. Peel them and remove the seeds. Set aside.
Gently brown the chopped red onion in a pan with olive oil. Add the tomatoes and season with a pinch of salt, black pepper and sugar if desired. Cook gently for 15 minutes; remove from the heat and stir in basil leaves. Set aside.
Boil spaghetti in plenty of boiling salted water. We recommend using 1 litre of water for every 100g of pasta. Before draining, remember to set aside a spoonful of pasta water to use for combining the pasta and sauce together.
Toss spaghetti in the pan with the tomato sauce, adding a little of the cooking water as needed. Sprinkle with grated Parmigiano and serve.