In a pan of boiling water, cook the large pasta shells for just 8 minutes, until almost cooked, but still firm and retaining their shape. Remove from the water and drain. When the shells are cold, carefully place in a large dish until you are ready to use them. Start making the salsa by heating up the olive oil in a pan. Add the garlic and onion and fry for a few minutes to soften. Add the tomato puree, chopped tomatoes, water, sugar, oregano and season to taste. Turn down the heat, and leave to simmer for about 15 minutes. Meanwhile, mix together the ricotta, eggs, 2 tablespoons of the grated parmesan cheese and the chopped parsley in a bowl. Season with salt and pepper, mix again, and place the mixture in a piping bag. You are now ready to put the dish together. Place 1/3 of the tomato salsa in the bottom of a large ovenproof dish. Take the pre-cooked pasta shells, and fill them with the ricotta in the piping bag, one at a time, putting them side-by-side in the dish on top of the salsa. When the shells are all filled, stuff a basil leaf into each pasta shell, covering the ricotta filling. Pour the remaining salsa over the pasta shells and sprinkle with the remaining parmesan cheese. Bake in the oven at 200`c, for about 20 minutes, till the pasta is golden brown. Serve the ricotta filled pasta shells on a large plate, side by side, sprinkled with some more grated parmesan cheese.