1. Preheat the oven: Preheat your oven to 190°C (375°F). Lightly grease a medium baking dish with butter.
2. Prepare the potatoes: Peel the potatoes and slice them evenly into approximately 3 mm (⅛-inch) rounds. Pat them dry with a clean kitchen towel.
3. Make the creamy sauce: Melt the butter in a saucepan over medium heat. Add the minced garlic and cook for about 30 seconds until fragrant. Stir in the flour and cook for 1 minute, whisking continuously.
4. Add the milk and cream: Gradually pour in the milk and heavy cream, whisking to prevent lumps. Cook for 3–5 minutes until the sauce thickens slightly. Season with salt, black pepper, and thyme.
5. Add the cheese: Stir in ½ cup of the cheddar and ¼ cup of the mozzarella until melted and smooth.
6. Layer the potatoes: Arrange half of the sliced potatoes in the prepared baking dish, slightly overlapping the slices. Pour half of the creamy sauce over them. Repeat with the remaining potatoes and sauce.
7. Add the topping: Sprinkle the remaining cheddar and mozzarella evenly over the top. Add Parmesan and paprika if desired.
8. Bake covered: Cover the dish tightly with foil and bake for 40 minutes.
9. Bake uncovered: Remove the foil and bake for another 20–25 minutes, until the potatoes are fork-tender and the cheese is golden and bubbling. If needed, bake for an additional 5–10 minutes.
10. Rest and serve: Let the potatoes rest for 10 minutes before serving. This allows the sauce to settle and makes serving easier.