Slice the potatoes into thin slices, about 3mm/⅛in thick. Place the slices into a bowl as you cut them.
Grate the garlic cloves on a grater. Scrape the grated garlic flesh into the bowl with the potatoes.
Season the potatoes, to taste, with salt and freshly ground black pepper.
Pour the cream over the potatoes and mix well again.
Place the potato slices into the gratin dish. They should come to just below the top of the dish. Press the potato down with the back of a spoon or your hands so it forms a solid layer. The cream should come to just below the top layer of potato (top up with more double cream if necessary).
Place the potatoes in the oven and bake for 1-1½ hours, or until the potatoes are completely tender. (If the cream looks like it's splitting, your oven is too hot, so turn it down a bit.)