Heat 1 tablespoon of olive oil in a large skillet and saute onion and 1 garlic clove for 3-4 minutes, until tender. Stir in the spinach and let wilt for 2-3 minutes, then stir in oregano and nutmeg. Remove from heat and let cool.
Once cool, chop the spinach mixture and put it into the large bowl. Mix in the ricotta, cooked chicken, and 1/4 cup of Parmesan cheese. Season with salt and pepper to taste. Use a piping bag or a teaspoon to fill the cannelloni tubes. Set aside.
Preheat oven to 350 degrees F. Lightly grease a 7x11-inch baking dish and set aside.
In the same skillet, heat the remaining teaspoon of olive oil, add 2 garlic cloves and cook for 1 to 2 minutes. Add diced tomatoes and Italian seasoning. Bring to the boil, lower the heat and simmer for about 10 minutes, stirring occasionally. Season with salt and pepper to taste and stir in the basil. Remove from the heat.
Spoon a thin layer of the tomato sauce over the bottom of a prepared dish and arrange the cannelloni in a single layer on top. Spread the remaining tomato sauce over the cannelloni to cover, then sprinkle with the remaining Parmesan cheese.
Bake for 30-35 minutes until bubbling and pasta is tender when pierced with a fork. Let rest about 5 minutes. Serve with some grated Parmesan and a few basil leaves, if desired.