In a large pan melt the butter at medium heat (be careful not to burn it), add the flour and whisk until it becomes a thick paste.
After a few seconds, add the warm milk and keep whisking until the milk starts to boil. Turn the heat to low and slowly keep whisking until the roux it's completely melted and the sauce starts to thicken.
When the sauce is thick enough to coat a spoon, season with salt, pepper and a pinch of nutmeg to taste. Remove from heat and set aside.
Make the filling:
Peel and cut in half a garlic clove, then drizzle some olive oil in a pan and fry the garlic for a couple of minutes.
Add the spinach (either fresh or frozen) and cover the pan with the lid. Mix with a wooden spoon a few times until they start to reduce in size (if necessary, add a few tablespoons of water to the pan).
When the spinach are fully cooked, season with salt, black pepper and a pinch of nutmeg. Drain them well and finely chop them. Set aside to cool down.
Once the spinach are cooled, mix them with the fresh Ricotta. Add 2-3 tablespoons of Parmigiano cheese and season again to taste.
Assemble the Cannelloni:
Spread 2-3 tablespoons of Béchamel sauce on the bottom of the baking dish. Pour the filling in a pastry bag without the tip and fill the Cannelloni pasta from top to bottom.
Place the stuffed pasta in the baking dish.
Roughly chop the Mozzarella and scatter all over the top of the pasta.
Pour the white sauce on top of the Cannelloni, covering completely the pasta also on the sides (otherwise it will not cook).
Evenly sprinkle a couple of tablespoons of Parmigiano cheese on top to make a nice crust.
Bake in the pre-heated oven at 200°C (400°F) for about 30 minutes (check also the pasta packaging for precise instructions as each pasta brand is slightly different).
Serve immediately.
Notes
MAKE AHEAD: you can prep the whole baking dish with the filled cannelloni in advance and store it covered with plastic wrap in the fridge for 1-2 days. Then bake them in the preheated oven just before serving.STORAGE: You can store leftover cannelloni in an airtight container or covered in the fridge for up to 3 days. Or you can freeze them for up to 1 month. Thaw in the fridge and reheat in the microwave or in the oven.