This is a tasty variation on a standard potato gratin.
Pepper (to taste)
Start by pealing and slicing your sweet potatoes.
Next, add them to a large pot with enough milk and cream to just cover them. Add several smashed cloves of garlic, several sprigs of thyme, several sage leaves, a knob of butter, and half a few dashes of nutmeg. Season with salt and pepper, and bring everything to a simmer (not a boil). Cook this enough for the milk and cream to become infused with flavor and thicken a bit.
Remove the slices of sweet potato from the liquid (but keep the liquid), then arrange the slices in a baking dish. To do this, add a layer of sweet potatoes, followed by grated parmesan and grated Gruyere. Repeat this until you’ve reached the top, then strain your milk and cream (removing the solids) carefully into your dish. The goal is to get the liquid just at or under the surface of your top layer of potatoes. If you need a little more liquid as well, you can bring a bit more milk and cream to a simmer with your herbs and garlic and add it to your dish.
Finish the top layer with another layer of Parmesan and Gruyere, then bake it at 350 degrees for approximately 45 minutes to 1 hour.
It’s also a good idea to let this sit for 30-40 minutes when it’s done cooking, to allow it to set a bit.