Preheat oven to 185°C (165°C fan-forced). Place sliced potatoes and onion in a large mixing bowl. Add cream and 1 cup cheddar. Toss until well coated and season.
Grease a shallow baking dish. Spread potato mixture evenly on baking dish. Top with remaining cheddar and Grana Padano. Bake for 50 mins or until tender and the top is golden brown. Serve straight from the oven.