Preheat the oven to 350°F and coat a 9x13 or 3 quart casserole dish with cooking spray or vegetable shortening. Set aside.
In a medium saucepan over medium heat, melt the butter and add the garlic, softening for 1 minute, but stirring to prevent browning.
Whisk in the flour to form a paste. Whisk in the cream cheese, allowing it to start melting, approximately 2-3 minutes.
Continue whisking and add the milk, cream, 3/4 cup of the parmesan cheese, 1 cup of the mozzarella cheese, onion powder, salt and white pepper until the sauce is fully blending and no large pieces of cream cheese remain. It will be thick. Remove from the heat.
Place the pasta into the prepared baking dish, top it with the cooked chicken. Pour the cheese sauce over the pasta and chicken, trying your best to cover all the pieces and using a fork or spoon to move it around to make sure it all gets covered.
Bake, uncovered, for 20 minutes. Remove and top with the remaining 1/4 cup of parmesan cheese and 1/2 cup of the mozzarella cheese. Continue to bake for 5-7 minutes or until cheese are melted and gooey.
Remove and allow to sit for 5 minutes before serving. Top with parsley, if desired.
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