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Heat the oven to 180C/160C fan/gas 4 and butter a shallow baking dish (I used a 20 x 30cm dish). Pour the milk and cream into a large saucepan, add the garlic, tip in the potatoes and bring to the boil. Remove from the heat to let the garlic infuse and the potatoes cool a little while you cook the bacon.
Heat the oil in a frying pan and add the lardons. Cook on a low-medium heat for 10 mins until they are turning crispy.
Using a slotted spoon, start layering the potatoes onto the base of the dish until completely covered (remove the garlic cloves when you spot them). Spoon over some of the cream mixture, a little cheddar, parmesan and gruyère, and scatter on some of the bacon. Season well and repeat until you have used up all the potato. Pour over any remaining cream and scatter with the remaining cheese. Reserve some of the bacon for topping later.
Cover with foil and cook in the oven for 1 hr. Remove the foil and cook for a further 30 mins until the potatoes are cooked through (insert a skewer or knife and check there is no resistance) and the topping is golden. Scatter with the remaining bacon for the final 5 mins of cooking.