Peel onion, garlic, celery and carrots. Press garlic through a garlic press. Finely dice onion, celery and carrots. Clean, finely dice and wash the leek. Heat the oil in a wide pot or pan.
Sauté the vegetables, onions and garlic thoroughly. Add the tomato paste, dust with flour and sauté. Deglaze with chunky tomatoes while stirring. Bring to the boil. Season with salt, pepper and 1 pinch of sugar.
Simmer for 10 minutes. In the meantime, prepare spaghetti in boiling salted water according to the instructions on the packet. Slice the parmesan. Wash basil and shake dry. Pluck the leaves from the stalks, except some for garnishing, and chop finely.
Add basil to the sauce just before serving and fold in. Arrange pasta and sauce on plates. Garnish with basil and parmesan and sprinkle with pepper.