Heat a medium saucepan (or skillet) over medium to medium-high heat. Add butter and cook until melted.
Add garlic and cook for 1 to 2 minutes stirring constantly to prevent burning. Add flour and stir to combine. Cook 1 minute.
Add the cream cheese and stir to combine. Cook until the cheese has become very soft, about 3 to 5 minutes.
Add the cream and Parmesan cheese; whisk to combine until the cream cheese is dissolved into the sauce.
Cook, whisking occasionally until the sauce begins to bubble and the Parmesan is melted, about 3 minutes. Reduce the heat to medium-low and continue to simmer, stirring occasionally, until the sauce has thickened a bit. Season with salt pepper, and nutmeg, and then give the sauce a stir.
Pour about 1/4 of the sauce into the bottom of a 9-inch x 13-inch baking dish. Spread the sauce over bottom of the dish.
Add frozen ravioli to the baking dish and spread into an even layer. Top pasta with chopped spinach and chicken so the ravioli is covered.
Pour the rest of the sauce over the chicken and spread evenly around the baking dish. Sprinkle all over the top the casserole with mozzarella cheese.
Bake for 30 minutes, or until the sauce is bubbly and the ravioli is cooked. Turn your oven to broil on HIGH and cook for 2-3 minutes, or until the top of the casserole is golden.
Carefully remove from the oven. Sprinkle basil over the casserole. Let sit 5 to 10 minutes. Serve warm with extra Parmesan if desired.
You can save prep time by using a rotisserie chicken from the store or by making extra chicken during the week when you know you'll want to cook this pasta dish.
If you're in a big rush, 2 1/2 cup of store-bought Alfredo sauce can be used, but homemade is SO much better and it's super easy to make.