Preheat oven to 400 degrees. Grease a 13 by 9-inch baking dish.
Place potatoes in a pot and cover with water until potatoes are covered by an inch or two.
Bring potatoes to a boil over medium-high heat, then reduce heat to medium-low and simmer until very tender when pierced with a fork, about 15 - 25 minutes.
Meanwhile melt butter over medium heat. Add garlic and saute 30 - 60 seconds to remove harsh bite, set aside.
Drain potatoes, return to pot and mash well or pass through a potato ricer or food mill.
Add butter mixture to potatoes along with, milk, mozzarella, 3/4 cup parmesan, and 2 Tbsp parsley. Stir and thin with a little more milk if desired and season with salt to taste.
Pour mixture into prepared baking dish, spread into an even layer. Sprinkle remaining 1/4 cup parmesan over the top.
Bake in preheated oven 20 minutes, for better browning on top move closer to broiler and broil briefly (about 2 - 3 minutes.) during last few minutes of cooking.
Garnish with 1/2 Tbsp parsley and serve warm.
Recipe source: adapted from Food Network