Preheat your oven to 190°C (375°F).

Blanch the sliced cabbage in boiling water for about 5 minutes until slightly tender. Drain and set aside.

In a separate pot, bring the milk to a simmer and cook the sliced potatoes until just tender, about 5-7 minutes. Use a slotted spoon to transfer the potatoes to a plate, reserving the milk.

In the same milk used for the potatoes, stir in the sour cream or crème fraîche, garlic, nutmeg, and sage. Season with salt and pepper. Allow the mixture to simmer gently for a few minutes to blend the flavors.

Remove from heat and stir in most of the Gruyère and Parmesan cheese, reserving a small amount for topping.

In a greased baking dish, layer half of the cabbage, followed by a layer of half the potatoes. Pour half of the cheese sauce over the layers.

Repeat with the remaining cabbage and potatoes, and pour the remaining cheese sauce over the top.

Sprinkle the reserved cheese on top.

Cover the dish with aluminum foil and bake in the preheated oven for 25 minutes.

Remove the foil and bake for an additional 25 minutes, or until the top is golden and bubbly.

Let the gratin cool for a few minutes before serving to allow the layers to set.