1. Prepare the tomato sauce. Mix all the ingredients and cook for 30 minutes, or until thickened to a rich, silky sauce.
2. To make the filling, finely chop mozzarella and place in a large bowl with the ricotta. Finely grate in half the Parmesan and then season to taste.
3. Take level tablespoons of the mixture and roll it into balls. Place each ball in a basil leaf, then put each inside a conchiglioni shell
4. Reserving half a ladleful of the simple tomato sauce, spoon the rest into a 20cm x 30cm ovenproof dish. Lay the filled pasta shells on top in a single layer, then drizzle over the remaining sauce. Cover with tin foil and place in the oven for 35 to 40 minutes, or until golden and bubbling, removing the foil and tearing over the rest of the mozzarella for the final 15 minutes. Once ready, finely grate over the last bit of Parmesan and return to the oven for a final 2 minutes to melt before serving.