Prepare the Cannelloni: Cook the cannelloni tubes for 5 minutes in boiling water. Drain, rinse under cold water, and set aside to cool until they are manageable to handle.
Preheat Oven: Preheat your oven to 400°F (200°C).
Cook the Filling: Heat olive oil in a large skillet over medium-high heat. Add the diced onions and sauté for 2 minutes, then add the minced garlic and chopped chicken. Cook for another 4 minutes.
Stir in the milk and simmer for 2 minutes until most of the liquid has evaporated. Add the ricotta cheese and stir until well combined. Add the baby spinach and cook for another 2 minutes until wilted. Remove from heat and let the mixture cool slightly.
Stuff the Cannelloni: Spread the pasta sauce on the bottom of a 9×13 baking dish. Stuff each cannelloni tube with the chicken and ricotta mixture using a spoon, your hands, or a piping bag. Arrange the stuffed cannelloni in a single layer in the baking dish.
Prepare the Sauce: In a mixing bowl, whisk together the half & half, grated Parmesan, dried basil, dried oregano, ground nutmeg, salt, and pepper. Pour this mixture over the cannelloni.
Bake: Bake the cannelloni in the preheated oven for 18 to 20 minutes, or until the top is golden brown and the cream sauce has reduced.
Serve: Let the dish stand for 3 minutes to allow the sauce to thicken. Spoon the sauce over the cannelloni and serve warm.