In a small saucepan, sauté the garlic in the melted margarine until tender. Stir in the cornstarch, salt, and pepper, and then add the milk. Stir well, and cook over medium heat for 2 minutes, or until the mixture is thickened and bubbly. Remove the sauce from the heat and set aside.
In the bottom of a 2-quart casserole dish coated with cooking spray, arrange half of the potatoes, onion, and parsley. Top with half of the sauce, and then repeat layers with remaining potatoes, onion, parsley, and sauce.
Cover the casserole dish and then bake at 350 degres F for 45 minutes. Stir once during the baking time. Afterwards, uncover and bake for an additional 30 minutes, or until the potatoes are done.