1. Filling preparation: In a non-stick pan, cook the ground beef, breaking it up well. Season with salt and pepper, add the finely diced onion. Add frozen basil. Cook for 5 minutes, stirring occasionally. Set aside.
2. In another non-stick pan, melt the butter and add the still frozen spinach. Cook for about ten minutes, stirring. Season with salt and pepper. Pass the spinach through a food processor. Then add it to the meat. Mix well.
3. Add the ricotta and mix well with a spoon to obtain a smooth and creamy mixture. Grate 30g of Parmesan cheese and mix. Set aside.
4. Tomato sauce: In a saucepan, thinly slice the onion and brown it well in olive oil. When the onion is well browned, add the peeled and chopped garlic cloves. Let it brown slightly. Then add the chopped tomatoes. Add thyme, bay leaf, parsley, and basil. Season with salt and pepper and simmer covered for about twenty minutes. Remove the thyme and bay leaf and blend with an immersion blender. The sauce should be velvety but not too thick.
5. Proceed with the actual preparation of the cannelloni: Grease an ovenproof dish and spread a good layer of tomato sauce on the bottom of the dish. Bring a large pot of water to a boil. Cook 2-3 lasagna sheets at a time (no more, otherwise they will stick together). Cook for 4-5 minutes. Remove from the water and place the partially cooked sheets on a clean towel. Cook 3 more sheets, regularly checking that the top sheet is well submerged in the water.
6. Place 2 tablespoons of filling on the first third of the lasagna sheet. Spread the filling widthwise to form a small roll, then roll tightly. Place the cannelloni in the dish covered with tomato sauce, making sure to place the seam side down so that it doesn't come undone during cooking and the filling doesn't escape.
7. Repeat the process with the remaining 12 lasagna sheets. Cover the cannelloni with the remaining tomato sauce. Don't hesitate to use a generous amount of sauce. This will allow the cannelloni to cook well and not become dry. In a preheated oven at 200°C (400°F), cover the dish with aluminum foil and bake for 30 minutes. After this time, remove the foil and grate the remaining Parmesan cheese. Bake for another 10 minutes until it is golden brown. Serve the cannelloni hot, alone or accompanied by a few green salad leaves drizzled with olive oil and vinegar.