Preheat oven 180c grease gratin dish (23x23) with butter. Slice courgette 0.2 thick using a mandolin or knife set aside. Slice the potatoes and keep separate

In a large pan heat the cream,milk and garlic until simmering and the potatoes return to simmer for 4 minutes season with salt and pepper.

With a slotted spoon strain out 1/2 the potatoes and arrange in a layer in bottom of the dish. Add 1/2 the cream mixture, then cover with 1/2 the courgettes by overlapping. Scatter with 1/2 cheese grated. Repeat with the remaining potatoes cream and courgette. Press down so the cream seeps up and cover with remaining cheese.

Cover with foil and bake for 1hr 15mins until knife slides easily. Remove foil and bake for another 10mins. Cool for 5mins before serving.