While the tomato sauce is simmering, cook the cannelloni tubes for 5 minutes in boiling water. Drain in a large colander, then rinse in cold water and set aside to cool for a few minutes, or just until cool enough to handle (see Notes).
Preheat the oven to 180C.
Heat the olive oil in a large frying pan or skillet over medium to high heat.
Add the onions and sauté for 2 minutes, then stir in the garlic and chicken and continue to cook for 4 minutes.
Add the milk and simmer for 2 minutes, or until most of the liquid has evaporated. Stir in the ricotta cheese until well combined. Then add the spinach and cook for 2 minutes, or until wilted. Season with salt and freshly ground black pepper, to taste.
Remove from the heat and set aside for a few minutes until it’s cool enough to handle.
Spoon 225g of the tomato sauce on the bottom of a large baking dish (approx. 23cm x 32cm) and set aside.
Using a spoon, your hands, or even a piping bag, stuff the prepared chicken mixture into the cannelloni tubes.
Arrange the cannelloni in a single layer in the baking dish, on top of the tomato sauce and set aside.