Par-cook the cannelloni. Cook the cannelloni tubes in boiling water for 5 minutes; rinse and set aside to cool for a few minutes, or just until cool enough to handle.
Prepare to bake. Preheat the oven to 400ºF.
Sauté. Heat the oil in a large skillet over medium-high heat. Add the onions and sauté for 2 minutes; stir in the garlic and chicken, and continue cooking for 4 minutes.
Make it creamy. Add the milk and let it simmer for 2 minutes, or until most of the liquid has evaporated. Stir in the ricotta cheese until well blended.
Add the spinach. Add the spinach and cook for 1 to 2 minutes, or until wilted. Remove from heat and let stand a few minutes, or until cool enough to handle.
Stuff the cannelloni. Spoon pasta sauce on the bottom of a 9x13 baking dish and set aside. Using a spoon, your hands, or a piping bag, stuff the prepared chicken mixture into cannelloni tubes.
Assemble. Arrange the cannelloni in a single layer on the bottom of the baking dish, over the pasta sauce. Set aside.
Make the sauce. In a mixing bowl, whisk together the half-and-half, parmesan, basil, oregano, nutmeg, salt, and pepper until thoroughly combined. Pour the half-and-half mixture over the cannelloni.
Bake. Transfer the baking dish to the oven and bake for 18 to 20 minutes, or until the top is golden brown and the cream is reduced. Remove from the oven and let stand for 3 to 5 minutes, or until the sauce thickens slightly. Spoon the sauce over the cannelloni and serve.
Finish and serve. Remove from the oven and let it rest for 3 to 5 minutes, or until the sauce thickens slightly. Spoon the sauce over the cannelloni and serve.