Follow these steps to bring this gratin to life:

1. Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with the unsalted butter.
2. In a saucepan, combine the heavy cream, whole milk, and minced garlic. Heat gently over medium heat until warm, but do not let it boil.
3. Layer half of the thinly sliced potatoes in the prepared dish, overlapping them slightly. Season with salt and pepper.
4. Sprinkle half of the Gruyère and cheddar cheese over the potatoes, followed by half of the thyme and rosemary.
5. Pour half of the cream mixture over the cheese and potatoes, allowing it to seep through the layers.
6. Repeat the layering process with the remaining potatoes, cheeses, herbs, and cream mixture.
7. Cover the dish with foil and bake for 45 minutes. Remove the foil and bake for an additional 15 minutes, or until the top is golden and bubbly.
8. Let it cool slightly before serving. This resting time allows the flavors to meld together beautifully.

Remember, the key is to slice the potatoes uniformly for even cooking. If you have a mandoline slicer, it can be a real time-saver.