Thoroughly wash the spinach in cold water. Heat a generous amount of olive oil in a skillet along with the garlic clove. Once the garlic is golden, discard it and cook the spinach for a few minutes until wilted.
Once cooked, transfer the spinach to a bowl with the ricotta, nutmeg, egg, salt, and Pecorino. Mix thoroughly until you achieve a thick, smooth filling.
Fill the cannelloni tubes with the ricotta and spinach mixture.
Spread a layer of tomato purée and béchamel sauce over the bottom of a baking dish, then arrange the stuffed cannelloni side by side.
Cover the cannelloni with the remaining tomato sauce, a light layer of béchamel, and a generous dusting of grated Pecorino cheese.
Bake in a preheated conventional oven at 200°C (400°F) for 40 to 45 minutes, checking occasionally, until a nice golden-brown crust forms on top.
