Pre-heat the oven to 190 degrees or gas mark 5. Peel and slice the potatoes into slices (about 2cm thick).
Chop the garlic and onion. Pour the milk and cream into a saucepan. Add the garlic and chopped onion.
Heat the mixture on a medium heat and as soon as you start seeing bubbles forming (it reaching boiling point), remove from heat and leave to the side - keeping it warm still.
Make your first layer of potatoes by placing the slices on the bottom of the dish. Ensuring to overlap them slightly. Pour your mixture on top and sprinkle some of the cheddar and Parmesan on top.
Repeat this step until your potato slices have run out, this recipe should make about 2-3 layers depending on the size of the dish.
Remember to put a layer of cheese on top too. Bake in the oven for 45 minutes and enjoy!