Pre-heat the oven to 180°c / gas 4. Grease a 30 x 20cm rectangle dish with butter and set aside.
Bring a large pan of salted water to a boil and par boil the potato slices for around 10mins, drain and set aside.
Melt the butter in a medium saucepan, add the onion and garlic and cook over a medium heat until softened.
Add the double cream, season with salt and plenty of black pepper and simmer for 5mins.
Take the pan off the heat, add the potatoes and mix together.
Pour into the dish and level. Bake in the oven for around 30mins until the potatoes are soft and the top is golden brown.
Remove from the oven and serve.
