Cook the spinach in a large dry skillet over medium-high heat, stirring, until wilted and all the liquid has evaporated, about 4 minutes. Transfer to a colander and let it cool enough to handle.

Gather the spinach in a clean kitchen towel and twist firmly to squeeze out as much water as possible. Chop the squeezed spinach finely.

In a large mixing bowl, combine the ricotta, chopped spinach, grated Parmigiano, egg, salt, pepper, and nutmeg. Mix until uniform and smooth. Taste and adjust salt. Set aside.

Warm the olive oil and garlic in a saucepan over medium heat until the garlic is pale gold and fragrant, about 2 minutes. Remove the garlic.

Add the crushed tomatoes, salt, and basil. Simmer uncovered for 12 to 15 minutes until the sauce thickens slightly and smells sweet. Taste and adjust salt.

Melt the butter in a medium heavy-bottomed saucepan over medium heat. Add the flour and whisk constantly for 1 minute until the roux is pale and smells faintly nutty.

Pour in the warm milk gradually, whisking continuously to prevent lumps. Cook, still whisking, for 5 to 7 minutes until the sauce coats the back of a spoon. Season with salt and nutmeg. Remove from heat.

Heat the oven to 190 C / 375 F.

Spread half the tomato sauce in an even layer over the bottom of the baking dish.

Fill the piping bag with the ricotta-spinach mixture. Pipe the filling into each dry cannelloni tube until full but not bursting. Lay the filled tubes in a single layer over the tomato sauce.

Spoon the remaining tomato sauce over the cannelloni, then pour the béchamel evenly over the top. Scatter the remaining 30 g Parmigiano over everything.

Cover the dish tightly with aluminum foil. Bake for 25 minutes, then remove the foil and bake for a further 12 to 15 minutes until the top is golden and the edges are bubbling.

Remove from the oven and rest for 10 minutes before serving so the layers firm up and portions hold together.