Preheat oven to 350°F. Spray a shallow baking dish with cooking spray. In large skillet, melt butter, saute onions until soft.
In baking dish, arrange in layers the sliced potatoes and onions. Sprinkle with garlic and seasonings in between layers.
Bake, uncovered for 40 minutes. Add whipping cream over all in baking dish. Sprinkle with paprika. Increase oven temp to 400°F. Bake for another 15 min or until potatoes are soft and gratin is slightly browned on top.