Prepare the Potatoes: First, preheat your oven to 180°C (160°C fan/Gas Mark 4). Peel the potatoes and slice them very thinly, about 2-3mm thick, using a mandoline if you have one – this ensures even cooking and a beautiful layered look. As you slice, immediately place them into a bowl of cold water to prevent browning. Once all are sliced, drain them thoroughly and pat them dry with a clean tea towel. This step is crucial for achieving that lovely crisp edge and preventing a watery gratin.

Infuse the Cream Mixture: In a large saucepan, combine the double cream, full-fat milk, crushed garlic, chopped fresh thyme, freshly grated nutmeg, salt, and black pepper. Bring the mixture to a gentle simmer over medium heat, stirring occasionally. You'll notice a fragrant, aromatic steam rising from the pan, and tiny bubbles will just begin to break the surface – this indicates the flavours are melding beautifully. Remove from the heat immediately to avoid boiling.

Layer the Gratin: Grease a large ovenproof gratin dish (approx. 25x35cm) generously with unsalted butter. Arrange a single layer of potato slices neatly in the bottom of the dish, slightly overlapping them. Ladle a portion of the infused cream mixture over the potatoes, ensuring they are well coated. Sprinkle with a handful of grated Gruyère and Parmesan. Repeat this layering process until all the potatoes and cream mixture are used, finishing with a generous layer of cheese on top. The layers should feel snug but not overly compressed, allowing the cream to bubble through.

Initial Bake (Covered): Cover the gratin dish tightly with foil. Place it into your preheated oven and bake for 45-50 minutes. During this time, the potatoes will absorb the creamy liquid and begin to soften. You'll notice a gentle bubbling sound from the oven, and if you peek, the cream will look visibly thicker and the potatoes will start to become translucent around the edges.

Uncover and Brown: Remove the foil. Increase the oven temperature to 200°C (180°C fan/Gas Mark 6). Continue baking for another 25-30 minutes, or until the top is a deep, glorious golden-brown and the cheese is bubbling vigorously and looks irresistibly crispy. To check for doneness, insert a thin knife or skewer into the centre of the gratin; it should slide in with very little resistance, indicating the potatoes are perfectly tender. The kitchen will be filled with a rich, savoury aroma that will make your mouth water!

Rest and Serve: Once baked to perfection, remove the gratin from the oven. This is perhaps the hardest step, but don't skip it! Let it rest for at least 10-15 minutes before serving. This resting period allows the gratin to set, preventing it from collapsing when you serve it and ensuring those beautiful layers remain intact. The internal bubbling will subside, and the gratin will firm up slightly, ready for elegant portioning.