In a frying pan, saute onion in 2 tablespoons of oil for 5 minutes. Add tomatoes, cover and cook slowly over very low heat until soft about 20 minutes. Add basil and stock and raise the heat to high. Simmer until the sauce has a good consistency about 10 minutes. Season to taste with salt and pepper and cover to keep warm. In another frying pan, saute leeks in remaining oil until well browned and starting to crisp. Drain on paper towels. Cook pasta as directed. Drain and mix into sauce. Serve topped with leeks and parmesan cheese.