Grease a 9x13 baking dish. Preheat oven to 350 degrees.
Add the peeled & cubed potatoes to a large pot. Add enough water to cover the potatoes and bring to a boil. (I add some salt to the water once it starts to boil to bring out the flavor of the potatoes.) Reduce heat to medium and continue to simmer for 18-20 minutes or until the potatoes are fork tender.
Drain the water and mash the potatoes with a potato masher.
Add the cream cheese, butter, sour cream, milk, garlic powder, salt, and pepper. Stir to combine.
Add 1 cup of the cheddar cheese & the parmesan cheese. Stir to combine.
Place the potato mixture into the prepared baking dish. Top with the remaining cheddar cheese.
Bake for 20-25 minutes or until heated through and the cheese on top has melted.
Garnish with the green onions.