Heat the oven to 200°C (180°C in a fan oven), 400°F, gas 6 and grease a baking dish with butter.
Dry roast the pine nuts, remove from the pan, chop and set aside.
Fry the garlic and the onion in hot oil. Add the spinach and allow to wilt.
Season with salt, ground black pepper and nutmeg.
Remove from the heat, leave to cool and then chop.
Mix the spinach with the pine nuts and Gorgonzola.
Fill the cannelloni with the spinach mixture and place them in the prepared baking dish.
Melt the butter in a pot and stir in the flour.
Add the milk and whisk together to remove any lumps.
Add the cheese and season with salt, ground black pepper and nutmeg.
Pour the sauce over the cannelloni and bake for around 30 minutes.
Remove from the oven and serve.