Preheat the oven to 180°C (350°F).
Peel and thinly slice 1 kg of potatoes (main) using a mandoline or a sharp knife.
In a saucepan, heat 500 ml of full-fat milk (main) and 250 ml of fresh cream (main) over medium heat. Add 2 crushed garlic cloves, 1 teaspoon of salt, and 1/2 teaspoon of black pepper. Stir well and bring to a simmer.
Add the sliced potatoes to the saucepan, ensuring they are well-coated with the milk and cream mixture. Cook for 5-7 minutes, stirring occasionally.
Grease a baking dish with 1 tablespoon of butter (main). Layer half of the potato mixture in the dish, followed by 100 grams of grated cheese (main) (preferably Gruyère or Cheddar). Repeat with the remaining potatoes and cheese.
Pour the remaining milk and cream mixture over the layered potatoes and cheese.
Bake in the preheated oven for 45-60 minutes, or until the potatoes are tender and the top is golden brown. Allow the gratin to rest for 10 minutes before serving.