Preheat oven to 350F. Butter a casserole dish, no larger than 9x13" in size. Set aside.

Peel and thinly slice the potatoes to 1/8" thick.

In a saucepan over medium heat, add the butter. Once melted, add the sage, rosemary sprigs, thyme sprigs, and smashed garlic. Cook until fragrant, stirring occasionally to prevent any burning, about 1-2 minutes.

Add the heavy cream, milk, salt, black pepper, and ground nutmeg. Stir to combine. Let simmer over LOW heat for 8-10 minutes. Remove from heat to cool to room temperature.

Add a single layer of thinly sliced potatoes overlapping one another in the buttered baking dish. Add a second layer of the overlapping potatoes.

Remove the rosemary and thyme sprigs from the milk mixture, then carefully pour it over the potatoes.

Cover with foil to bake for 1 hour 20 minutes.

Remove foil, evenly distribute the grated gruyere on top, then bake an additional 20-25 minutes. It should be bubbly and golden brown on top when ready.

Let rest at least 15 minutes before garnishing with fresh thyme and cracked black pepper to enjoy warm!