Grease a 1 1/2 quart casserole dish, set aside. Peel the potatoes and quarter them. Cook them in salted water for 20-25 minutes or until they are tender; drain.
Meanwhile, in a small skillet cook the onion and garlic in the butter until they are tender. Mash the drained potatoes with a potato masher. Add the onion and garlic mixture, sour cream, salt and pepper to your liking. Gradually mix in enough milk to make creamy. Transfer to the casserole dish. Cover and chill for about 1 hour.
When you are ready to serve, sprinkle the top with parsley and put in a preheated 350 degrees F oven for about 50 minute or until they are heated through. The top will get a light color brown and that is what you want.