Peel and cut the potatoes, place in a large pot filled with water and a dash of salt. Bring to a boil then reduce to a med-high, let cook for at least 20 minutes or until you can easily pierce the potatoes with a fork.
Preheat oven to 350F, spray a casserole dish with some non stick spray *I used a 9x9 dish
Drain the potatoes, add the milk and butter then using either an electric or hand masher mash them til they are nice and creamy.
In a bowl beat together the cream cheese, sour cream, seasonings, garlic, 2/3 cup cheddar cheese and 1/4 cup of parmesan cheese (the remaining cheddar and parm will be sprinkled on top) stir this mixture into the mashed potatoes, don't over mix it though, just gently fold it in. If you can add the dairy ingredients at room temp it will give you a creamier potato.
Scoop the potato mixture into the casserole dish. Sprinkle the remaining cheese on top (1/3 cheddar and 2 Tbsp parm) cover with foil, best to spray the foil to prevent potatoes from sticking.
Bake in the oven at 350F for 20 minutes, remove foil and bake another 5 minutes, feel free to top with some fresh green onion.
Note* the 8 medium potatoes worked out to be about 6 cups of cooked potatoes, according to the WW app this is 24SP