Heat the oven to 200°C (180°C fan). Grease an oven- and stove-proof dish with some of the butter, saving the rest. Finely chop or grate the garlic and rub it all around the dish. Wash or peel the potatoes and slice them about 1cm thick. Lay one layer of potatoes in the dish and season with salt, pepper and thyme leaves. Repeat until all the potatoes are used. Pinch the leftover butter into small pieces, scatter over the top and pour over the milk. Simmer on low heat for 20 minutes until the potatoes start to become tender, pushing them down a few times so the milk covers them. Grate the cheese, sprinkle it over the top and bake for 20 minutes. Rest for 10 minutes before slicing.