Sweat the bacon in the olive oil over a medium heat for two minutes in a large-ish pot. Add the leeks and garlic and cook for another two minutes.
Meanwhile, slice the potatoes to the width of a two euro coin, you could use a food processor for this, but if the potatoes are cut thicker, they could take a little longer to cook.
Preheat the oven to 170C. Add the potatoes, cream and milk and bring to a simmer. Season, add the parmesan and mix well.
Transfer to a large gratin dish. Press the mix down so it is all covered with the cream and bake for 15 mins. Press down once again at this stage with a fish slice to make sure all the potatoes covered with the cream. Cook for a further 15-20 mins until golden brown and cooked through. If a knife slips easily into the potato, it is done.