Cook the conchiglioni for seven minutes in salted boiling water then strain, quickly cool, and set aside.
In a pan gently sweat the chopped shallots with the butter. Add the roughly chopped spinach, and cook until all the water has gone. Add the chopped herbs and cook for a further couple of minutes. Remove from the heat and leave to cool slightly. Add the ricotta, grated grana padano, and egg, a little freshly ground nutmeg, and season with salt and pepper. Gently mix to combine.
To make the tomato sauce, gently heat one tablespoon of the oil in a heavy-bottomed pan. Add the onion and sweat until translucent. Add the tomatoes, cover with a lid and leave to cook for one hour and 20 minutes. In a separate pan, fry the garlic in the remaining oil until golden. Add the basil and combine with the tomatoes. Cook for a further 20-30 minutes. Season and pass through a mouli or rub through a coarse sieve.
Pre-heat the oven to 180°C. Fill the conchiglioni shells with the ricotta and spinach mix, and place them in a deep baking tray. Pour 300g of tomato sauce over the top, and sprinkle generously with freshly grated grana padano and little dollops of butter. Bake for about ten minutes, then serve.