Defrost and drain the spinach. Drain the tomatoes. Collect 1 tablespoon of oil. Peel garlic and onion and chop finely. Heat the oil in a saucepan. Sauté garlic and onion over medium heat until translucent.
Add tomatoes and stock and simmer covered for about 6 minutes. Cook pasta in boiling salted water for about 5 minutes until al dente. Roast pine nuts in a pan without fat until golden brown. Coarsely chop the spinach.
Cut the drained tomatoes into pieces. Mix spinach, pine nuts, tomatoes and ricotta. Season to taste with salt, pepper and a little sugar. Drain the pasta and rinse with cold water. Season the sauce with salt, pepper and a little sugar and remove from the heat.
Fill the cannelloni with the spinach-ricotta mixture (cut a large corner from the piping bag or fill by hand). Cut filled cannelloni in half crosswise with a sharp knife. Put the tomato sauce in a suitable casserole dish.
Place cannelloni vertically close together in the mould. Grate the mozzarella coarsely and sprinkle over it. Bake in the preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas: level 3) for 20-30 minutes until golden brown.
Remove the casserole, allow to cool briefly and garnish with basil.