Preheat the oven to 390°F.

Bring a pot of water to a boil. Immerse the tomatoes in the boiling water for 10 seconds, remove them, and immediately cool them under cold running water. Peel off the skins, cut the tomatoes open, remove the seeds, and chop the flesh into pieces.

Peel and finely chop the onion and garlic. Cook the spinach and then squeeze it out well. Beat the eggs.

In a bowl, mix the ricotta with the beaten egg and the well-squeezed spinach. Season with salt, pepper, and ground nutmeg.

Fill the cannelloni with the ricotta mixture — this can be done with a piping bag, a sturdy deep plate and a teaspoon, or with a small spoon that you place into the tube and push out. Butter a baking dish and arrange the filled cannelloni side by side in a single layer.

Heat a splash of olive oil in a pan over medium heat. Sauté the onion and garlic until soft and translucent. Add the prepared diced tomatoes and let everything simmer gently for about 5 minutes so the flavors blend. Season with salt and pepper.

Melt the butter in another pan over low to medium heat. Add the flour and stir for about 1 minute so the flour taste disappears (roux). Gradually add the milk, stirring continuously with a whisk to avoid lumps. Let the sauce simmer gently until it thickens to a creamy consistency. Season with salt and pepper and stir half of the grated Parmesan into the sauce.

First spread the stewed tomatoes over the filled cannelloni in the baking dish. Then pour the béchamel sauce evenly over the cannelloni. Sprinkle with the remaining Parmesan.

Bake the dish for about 30 minutes in the preheated oven at 390°F. After baking, let the dish rest for a few minutes before serving so the sauces can set slightly. Enjoy!