Preheat oven to 180C. Place the cream, milk and garlic in a medium bowl. Stir to combine. Season to taste.
Arrange one-third of the potato, overlapping slightly, in a single layer in the base of a 5cm-deep, 33cm x 22cm rectangular baking dish. Season. Pour over one-third of the cream mixture. Sprinkle with one-third of the cheese and half the salmon. Repeat twice to form three layers, finishing with potato, cream mixture and cheese.
Cover with foil and bake for 45 mins. Uncover and bake for a further 30 mins or until the potato is just tender and the gratin top is golden brown. Set aside to cool slightly before serving.
