The Preparation
Preheat oven to 325°F.
Slice potatoes thin (about 1/8 inch) using a mandoline or sharp knife, cutting lengthwise.
In a saucepan, melt butter over medium heat. Saute shallot until softened. Add garlic and thyme and cook 1 minute more.
Add heavy cream and milk, season with salt, white pepper, and a pinch of nutmeg. Bring to a gentle simmer.
Layer half the potato slices in a greased baking dish, slightly overlapping.
Pour half the cream sauce over the potatoes. Sprinkle with half the Gruyere and Parmesan.
Add remaining potato slices and pour the rest of the cream sauce over the top.
Top with remaining cheeses.
Bake at 325°F for 1 hour 15 minutes, until potatoes are tender and top is golden.
If needed, broil for 2-3 minutes at the end to get a deeply golden, bubbly crust.
Let rest 10-15 minutes before serving.
