Heat oven to 350. Spray a 3-quart casserole dish w/ cooking spray. Melt butter in lg skillet over medium heat. Add onion, cook about 5 mins, stirring occasionally.
Add the milk, cream and 1/2 tsp pepper. Bring to a low simmer. Add cream cheese. Cook, stirring until melted. Stir in 1/2 cup parmesan cheese, parsley and thyme.
Place potatoes in casserole dish. Pour milk mix over potatoes and sprinkle w/ 1/4 cup parmesan cheese. Cover casserole dish w/ foil. Place casserole dish on a lg baking sheet. Bake until potatoes are tender, about 1 hr. Sprinkle w/ chives.