Preheat oven to 400° F.
Slice the potatoes and turnips and set aside in a large bowl.
In a medium, non-stick skillet, heat 1/2 tablespoon butter over medium heat. Add the diced onions and garlic, saute until translucent.
Add the cream cheese, heavy cream, salt and pepper to the skillet. Bring to a simmer and stir until smooth.
Turn off heat.
Spray a 9x13 baking pan with non-stick cooking spray.
Put half of the sliced potatoes and sliced turnips into a separate large bowl.
Slowly add ⅓ the cream mixture into the bowl with the potatoes and turnips until coated well.
Using your hands place the coated potato and turnip slices into the baking pan, vertically, making sure the slices are close together.
Add another ⅓ of the cream mixture to the remaining potatoes and turnips, coat well. Layer them into the baking dish.
Once all the slices are in the baking pan, pour the remainder of cream mixture into the baking pan.
Cover the pan with aluminum foil and place in the oven for 30 minutes.
After 30 minutes remove the foil and bake for an additional 40 minutes.
Remove from oven, sprinkle on Parmesan cheese and bake for an additional 15 - 20 minutes. Sprinkle on chives right before serving if desired.
Enjoy!